If you've only ever stitched on aida, you might be confused when a chart talks about stitching "over one" or "over two". These terms usually are used when stitching on linen and evenweave fabrics.
Stitching "over one" means you're moving your needle from one hole to the next (moving over one), such as this:
Over-one stitching is also usually done with one strand of floss. Sometimes, stitches will slip to the backside of the fabric. To avoid this, complete each X before starting the next.
In contrast, "over two" is explained HERE.