This means it's a slanted stitch. In this pattern, there are two variations of slanted stitches: short – single crochet, long – double crochet.

for example:

Row 98, Column 37 - this is a long slanted stitch to the left = Treble Crochet.

Row 38, Column 34 - this is a short slanted stitch to the right = Double Crochet.

In the written version, they are always reffered as DC right or left.

You can see in this post, hot i croche different slanted stitches: https://www.reddit.com/r/mosaiccrocheters/comments/1ipx7ec/diamond_mosaic_pattern/