In crochet, it’s a convention that at the beginning of a row or round, where you need to get to the height of the next row, you chain a certain amount of stitches to replace the stitch.  The relationship goes something like this:

Sl st- 0 chains

Single Crochet – 1 chains

Half Double Crochet – 2 chains

Double Crochet – 3 chains

Treble Crochet – 4 chains

Double Treble Crochet – 5 chains

… and so on a soforth.

The key is, you can not only substitute chains for a stitch, but you can substitute stitches for chains.

Take this granny square.  It’s pretty standard, but you see where the single crochets are (hint: The red x’s)  They are worked into the 3rd chain.  I do this instead of doing a slip stitch to finish off the row.

Confused?

Let’s see if I can explain it with pictures.

Things making more sense now?  Let’s compare the two different types of Granny Squares.