What does decrease mean in crochet instructions?

Answer 1

It means decrease a stitch. You do this by putting two stitches together by leaving the last loop of a stitch open and completing it with the next stitch. For example, to decrease using single stitches:

Insert hook, yarn over, pull through (so far, a normal sc), yarn over, and pull through only one of the loops on hook.

Then, leaving the two loops on the hook, insert hook into next stitch, yarn over, pull through, yarn over, and then pull through all loops on hook.

This can be done with any stitch, simply leave one loop of the stitch on, then on the next stitch pull through all loops.