A crochet decrease makes the current row or round that you are working on shorter than the one previous to it. This video crochet tutorial will help you how to crochet decrease in all basic stitches.
The decrease (which is abbreviated “dec” in crochet) may also be called “crochet two together” since that is the essence of what you are doing. So your pattern might say “decrease” but it might also read something like “sc2tog”, which translates to “single crochet two together”.
You will make two single crochet stitches side by side in such a way that they are joined at the top and they become one.