Knitting tension or gauge is the number of stitches and rows per inch, and determines the size of the garment you are making. Knitting patterns specify the tension on which the sizing is based.
To ensure a proper fit, it is essential to knit to tension. Make a test swatch 5-6"square and measure the number of stitches and rows over 4"/10cm. If the number of stitches and rows does not match the pattern's tension, you must change your needle size.
If you have too MANY stitches use a LARGER needle. If you have too FEW stitches use a SMALLER needle. Try different needles until you get the proper tension.