What is this symbol: semicircles facing each other?

These are "solder links" and could look like:

Note how the top right one looks the most similar to the symbol from the schematic.

These are copper shapes on a PCB.

If you don't do anything these structures will be an "open" so they make no connection.

But using a soldering iron and some solder it is easy to apply some solder and make that connection.

This can be used to connect / disconnect certain parts of a circuit on a PCB and so set certain options, for example to feed a certain voltage to a line (or not).

Those are solder-bridge points and the circuit symbol accurately represents the shape of the PCB pads.

Figure 1. Options J1, J2 and J4 have been bridged. Option J3 has been left open. Source: PCB Libraries.