Knitting gauge and swatches
Gauge is about how much space the stitches you knit occupy. If you adhere to the gauge stated in your pattern, your finished work will be the size you're knitting for. It is, therefore, crucial to adhere to the gauge so that you don't end up investing a lot of time in something that won't fit.
When you check your gauge, you need to make a gauge swatch. The gauge swatch should always be in the same stitch pattern as specified in your pattern, such as stockinette or double rib. The gauge swatch should measure a little over 10 cm in both width and height, allowing you to measure 10 cm without including the corner stitches. If you're knitting with yarn or in a pattern that changes after washing, it might be a good idea to wash and possibly block your gauge swatch before measuring it. When measuring your gauge, count how many stitches there are in 10 cm.
If you have too many stitches in 10 cm, use a larger needle.
If you have too few stitches in 10 cm, use a smaller needle.
Once you've found the needle size to achieve the correct gauge, you know that it's the size needle you should knit with. Everyone's gauge is different, so you may not necessarily achieve the gauge with the same needle size as stated in the pattern.
When you adhere to the gauge, you know that your finished work will be the correct size. If not, your work may end up being too large or too small