There are many different types of filet crochet, most filet is done with blocks, spaces and posts. Most filet uses just two stitches to make the blocks and spaces and tr (dc USA) and chains.
A post is the tr (dc USA) that is on each side of a block or space.
A block is the tr (dc USA) X 1, 2, 3, 4 depending on the yarn, design and type of filet that fills and makes the pattern.
On the finished piece it is usually the solid part that makes up the design i.e. the heart on the finished filet piece. The black squares on the graph.
A space is the chain/ s that make the open mesh/ hole.
The white squares on the graph.
Depending on the type of filet, you will find you have
1 X 1 filet = 1 post and a single chain or single tr (dc USA) can produce a long thin image
1 X 2 filet = 1 post and 2 chains or 2 tr (2 dc USA)
1 X 3 filet = 1 post and 3 chains or 3 tr (3 dc USA)
1 X 4 filet = 1 post and 4chains or 4 tr (4 dc USA) can produce a short fat image
The designs on my site have been tested using 1 X 2 filet because I find it produces an image similar to the graph drawings, but you can use whatever filet type you prefer.
1 x1 filet is used for worsted weight and thicker yarns.
1 x 2 filet is used for thread weight yarns
1 x 3 (and very rare 1 x4) is not common now but was used with very thin silk thread weight threads