Crochet gauge is basically the number of your stitches per inch, and your rows per inch. I specify 'your' because gauge will change with the type of yarn, wool, or cotton you are using, the size of your crochet hook and your way of crocheting. If you are making a sweater and your gauge is tighter than the person's that created the pattern - your project will end up too small for you. Almost all patterns, that have a size, also specify gauge.
The temptation is great. You found a beautiful pattern, you carefully picked out your yarn, wool, or cotton and bought all your other supplies. All you want to do is get busy on creating your latest masterpiece. You don't want to do the boring part - checking your crochet gauge.
Stop and ask yourself, "Do I want to spend hours, maybe days, crocheting - only to learn that the end result is something I never use?"
Always check your crochet gauge.
Gauge is always checked using the main crochet stitch pattern of the project unless the pattern says otherwise. If its something simple, like Single, Double, Treble Crochet, etc., all you have to do is a little crocheting and measuring before you start. If there's a pattern to your stitches you need to crochet that grouping. Usually the pattern will tell you what to use to check your gauge - but if it doesn't, always remember to use the main stitch pattern.
The next thing to do is create a swatch that's over 4 inches wide and high. To get a perfectly accurate gauge you need to measure 4 inches - not just one - AND you should crochet something larger than 4 inches to make sure you have it correct.
Why? If you're gauge is 4 stitches per inch, and all you check is one inch, you may find your calculation isn't exact. At 4 inches you can really tell what the gauge is. I've counted 4 stitches in an inch - and yet, when I did 4 inches I only counted 14 stitches (that would be 3.5 stitches per inch), not the 16 that should be there. This told me to increase the size of my crochet hook to get the exact gauge for the project.
My standard swatch is 20 stitches by 20 rows. But if you're working with small hooks and thin yarn you may need to crochet more. A pattern stitch may also be different.
If my pattern called for 4 ply yarn, using a size G 6 / 4.00 mm crochet hook - and told me the gauge was 4 stitches = 1 inch and 4 rows = 1 inch ...
Although you can count 16 stitches you'll note my stitches are a little tight, and the bottom is closer together than the top (see below).
My rows are very off, 18 in 4 inches.
From this point I would know to use a larger crochet hook (most likely a size H). I would do another swatch and have a look. If I had just gone about making my project, it would be too small.
Also note that the very bottom is tighter than the center. If I had just done a small, one inch swatch my crochet gauge count would be even more off.
The most important lesson to remember here is to always do a swatch large enough to measure 4 inches.