I am just returning to crochet from a long absence, what does the * mean before a stitch instruction?

Usually Asterisks are paired, with one at the start and one at the end of a set of instructions. These delineate a pattern instruction that is to be repeated from * to * across a row or part of a row. You will also see the following punctuation marks used in a similar fashion--( ), { }, or [ ]. Any of these may delineate a repeat section, or a single instruction to be repeated. If followed by a number like ( 3 dc, ch, 3dc) 3 times, you will repeat the instruction within the parenthesis three times before continuing with the rest of the instructions. In patterns with different sizes you will also see parenthesis used to show a series of sizes where you need to know which number within them is ociated with the size you are making.