When I refer to good sides together that can refer to either cutting or sewing:
Cutting with good sides together is a technique that is used when two identical, or mirrored pieces need to be cut.
When a pattern asks to cut a piece with good sides together, fold your fabric double with the good side/right side of the fabric to the inside, and the bad side/wrong side outside.
Now mark the pattern piece on the bad side of the fabric, and cut both layers in one go.
This gives you two pieces that are mirror images of each other.
When cutting good sides together, make sure both sides are on grain.
Sewing with good sides together is the standard approach to making a seam.
By placing the good sides of the fabric together, both pieces will be joined with the seam allowance to the inside of the garment. This is what's done for the vast majority of seams.
When there's no specific instructions on how to construct a seam, it's good sides together.