What does this schematics symbol mean?
Taken from here.
I got the PCB and it's not a button.
It looks like a solder bridge. Sometimes there can be a jumper that you need to put, sometimes it is just two very close pads that you join with a blob of solder.
Either a jumper or a cuttable trace on the pcb (as opposed to a unpopulated trace). As an i2c module, it's so you can disconnect the on board i2c pullups if you have them somewhere else or if the values don't suit your need. A picture of the board would confirm, but I'm 110% on this.
It looks like jumper; The LED ON/OFF we configure through the this jumper.