Regolith - sswelm/KSP-Interstellar-Extended GitHub Wiki
Regolith (wikipedia) is the loose layer of material covering solid celestial bodies. It can be thought of as the 'soil' of the planet, although technically there are some differences between soil and regolith. It contains various useful elements that could be exploited in ISRU processes (wikipedia). Regolith on different celestial bodies can be rich in various elements that are rare on Earth due to the regolith there being bombarded by solar wind (charged particles emitted from the sun which Earth is mostly shielded from by its magnetosphere).
In KSP Interstellar-Extended, regolith is a resource that can be mined on solid bodies and then converted in the [ISRU refinery](wiki/ISRU Refinery) to many different useful resources. It can currently be mined only on bodies without atmosphere (so it can be mined on Mun in stock KSP star system, for example, but not on Kerbin or other bodies with an atmosphere).
Use of regolith
Regolith contains many useful components that can be processed in the [ISRU refinery](wiki/ISRU Refinery). Hydrogen, Carbon Monoxide and Carbon Dioxide, Methane, Nitrogen, Helium-3 and Helium-4 and even Water can be processed out of regolith at various rates. This makes it a very useful resource that can be exploited to provide fuel or life-support materials. To give you an idea about the abundance of the components in regolith, see the image below.
Mining regolith
Regolith mining is done by the regolith mining drill. The vessel it is on needs to be in contact with ground, on a planet with no atmosphere. The drill needs to be deployed and activated. It will collect regolith automatically afterwards and will do so even when the vessel is not in focus - so you can easily set up an automated mining base and come back to it much later. Be sure to provide proper cooling, as regolith drills produce waste heat. And supply the drills with the power needed (currently 5 MJs per drill - this can be tuned in the part config if you are so inclined).
Regolith abundance
Regolith is only available for mining on planets without atmosphere. There is some regolith sure to be found regardless of the specific location. But generally, the higher the terrain at your location, the more regolith is going to be there - so you might want to plop down your drill rigs in highlands rather than flats (though there's still going to be some regolith even at low altitudes). Additionally, each planet is bound to have some hotspots - places where the regolith is found in higher quantities. So you might want to use stock ore scanners (set so they're scanning for regolith) to find these and exploit them to your benefit. (But if you want to keep it simple, all you have to remember is - the higher your vessel is when landed, the better.)
Example mining operation
Let's say we want to mine the regolith on the Mun. Maybe it's going to be a part of our long-term Mun colonization mission.
The part above is the one you definitely need for mining regolith. The clue is in the name.
So let's build some basic regolith lander. Be sure to provide it with MJ power (e.g. reactor+generator combo, or perhaps beamed power receivers), proper cooling (remember, drills produce waste heat) and most importantly, some regolith holding tanks.
While you're preparing your mining rig, you might want to use the KSP stock ore detectors around your target planet (like on the tiny craft above), so that you know where would be the best spot to land. Set them to detect regolith, let them draw overlay and set the cutoff point to some reasonable percentage so that it's easy to see the hotspots.
The regolith distribution on Mun in this particular game looks like this. While there will be regolith everywhere we land, there are several hotspots.
Once your lander is in orbit (I leave the launcher and delivery to target orbit to your imagination), pick a good landing spot.
Preferably somewhere with a good view!
Before you activate your drill, you have to extend it. Don't worry - if you forget, the drill will remind you.
So here we are, with both drills extended. All that's left is to activate them.
And there we have it - the first bits of regolith have been succesfully drilled. But see the rate? This is going to take a while. Thankfully, the regolith drill works when the vessel is out of focus as well! So feel free to leave the mining rig in this location as it is and go play with planes on Kerbin or plan your next missions. Once you come back to this vessel, you'll see it has been working hard on your behalf the whole time.
If you come back a few hours later, you might just find that the regolith tanks are full! Now it's up to you what you do with it - you could transport it to a nearby base and process it there, land a processing lander with an [ISRU refinery](wiki/ISRU Refinery) nearby and connect them up, you could have even made your drill rig have an ISRU refinery already built in, so that it would now essentially be a small regolith processing outpost on Mun etc. The possibilities are endless.