t-ch = turning chain
Stitches in crochet come in varying heights. When you begin a row of any stitch, you will be starting at the base of the row. In order to come up to the height of the stitch you will be working, you must start with a chain that is the same height as the stitches in the row you are working. This chain is called the turning chain. It can be worked before or after turning your work. The illustration above shows the most common stitches and the length of the turning chain for each stitch.
Single crochet: one chain
Half double crochet: two chains
Double crochet: three chains
Triple crochet: four chains
Usually, the turning chain at the beginning of the row, takes the place of the first stitch of the row. So, after working the turning chain, you will not work another stitch in the first stitch. And when you come to the end of the row, you will treat the turning chain as a stitch and work in it. For the single crochet, the rules change. The turning chain worked at the beginning of a single crochet row does not count as a stitch. After working the turning chain, you will also work a single crochet in the stitch below the turning chain. You will not work in the turning chain on the return row.