the midrib is the thicker stripe in a single moss leaf. It's better visible with a magnifier. OK, hardly visible in my pics...
Some mosses lack a midrib or have 2 short ribs in the base of the leaf, e.g. Java moss.
Leptodictyum riparium is widespread and frequent in the northern hemisphere and very variable but there are some other Leptodictyum species and I don't know how they differ. There are also more similar genera, e.g. Amblystegium (Leptodictyum is counted among Amblystegium by some authors, too).