Cross stitch is a way to work a design or pattern onto your crochet piece after the piece is complete. It is actually embroidery worked on the surface of the crocheted fabric. It is used to add color details, such as a stripe or small motif.
Using a series of X's, you can create a motif either by following a chart or working free-hand. This technique works well on felted pieces as well.
Thread a large, blunt-eyed needle with your yarn of choice. Starting from the back of the fabric, bring the needle up through the lower right corner of an existing stitch leaving about an inch of yarn for the anchor. Stitch downwards into the upper left corner of the stitch. (This makes half of your cross-stitch). Next, come up through the lower left corner of the stitch and down on the upper right. You should have an "X" on the right side of your fabric. As you continue your anchor should be stitched to the back of the fabric.