Synergy is a measure of how well the materials in a part are working together. The synergy value applies a multiplier to some of the part's stats. A single-material part will have 100% synergy, which adds no multiplier. By mixing materials, that value can be increased (or decreased if the materials are drastically different).

In older versions, synergy was calculated for the final gear item, based on the main parts only. In recent versions, synergy only applies to parts. The parts will contribute whatever stats they have to the final item (after synergy is applied).

How Synergy is Calculated

Synergy is first based on the number of unique materials (as in different type, not the number of items used in crafting). This has diminishing returns. Second, synergy is reduced based on the differences in tier and rarity, compared to the highest tier and rarity in the gear. With poorly matched parts, synergy can drop below 100%, weakening the crafted part.

(TL;DR) In other words, add different materials when crafting gear parts, but try to pick materials with similar tier and rarity. Matching tiers is easy enough, but odds are you will not be able to match rarity exactly most of the time.

Traits

Some traits may affect synergy, increasing or decreasing it depending on the value before traits are applied. Please see the trait list page and search for silentgear:synergy traits. Your mod pack's author may have provided a similar page on their own wiki. If not, they can easily generate one with a command.