What does this schematics symbol mean?
Taken from here.
I got the PCB and it's not a button.
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.