In Crochet: Each crochet stitch has 2 loops on the top of the stitch. The illustration above identifies these loops. Usually, you will insert your hook under both loops of the stitch in the previous row. Sometimes, to create a decorative effect, a pattern will indicate to work in either the front loop or back loop of a stitch.
In Knitting: (sometimes also written kbl for 'knit through back loop') Knit into the back of the stitch instead of the front of the stitch. This gives a tighter stitch and is sometimes used for ribbing.