Decrease means to reduce in number; in the knitting world it means to reduce the number of stitches you have on your needle. Decreasing is often required for shaping a garment, such as making an armhole of neck hole.
There are several different methods of decreasing, but the most popular are knitting (or purling) two stitches together and the SSK, or slip, slip, knit.
Knitting or purling two stitches together is performed exactly the same way you would knit or purl one stitch. Just put the right-hand needle through two loops instead of just one.
To make a slip, slip, knit, slip the first stitch onto the right-hand needle as you would to make a knit stitch, then slip the second stitch in the same manner. Using the left-hand needle, slip back through those two stitches and make a knit stitch, but don't slide it back onto the left-hand needle when you are done.
The type of decrease you will use will be explained in the pattern.
