Patterns are used in various commands when changing blocks. e.g. //set <pattern> and //material <pattern>

Use the name or id of a block (e.g. stone). For more info see this page on the block data syntax

You can use hand, or e.g. slot5 to use blocks from your inventory.

Use a comma (,) to randomly use a block from multiple patterns. For specific ratios you can use % e.g. 80%stone,20%planks

Pattern arguments should be inside square brackets e.g. #simplex[10][stone,wood]