A hatch is the pattern/color of materials in a drawing. Everything you "cut through" in your drawings must be hatched, everything you don't "cut through" you don't hatch.
An example:
Each material has it's own hatch. A few examples:
-Stone bricks will look like this: // // // // // //
-Insulation will look like this: /\/\/\/\/\/\/\/\/\/\
-Some types of wood will look like this: xxxxxxxxxxxxx