tbl = through the back loop
In Crochet: Each crochet stitch has 2 loops on the top of the stitch. 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.