This pattern is for a double filet/interlocking crochet afghan square which uses the ELCG (East London Crochet Group) format of both written instructions and a graph. This square was designed to introduce diagonals to those new to them in double filet, It also includes a separate work sheet, with photographs, to explain the new abbreviations DB and BD in this form of crochet.
If you have not tried this form of crochet before I would recommend trying a simpler square, like the ones in our 25 Years and counting first.
Although double knit is listed as the preferred yarn, you can make this square in any weight as long as you adjust your hook appropriately and we always recommend a tension square as double filet/interlocking crochet can cause you to tighten your tension. You will need two contrasting colours, I would not recommend a very dark colour for your first attempt at diagonals as sometimes it can make it very difficult to see the base of the stitches.