 If you have development or breakout boards, you've probably already seen PCB solder jumpers. There's sections of copper on the PCB that can be used for post-manufacturing configuration, for example, to change pin settings or i-squared C addresses. This board has both default closed and default open jumpers. Closed have a tiny little copper trace connecting the two larger pads, and the ones below here have no connection. Adafruit boards use this triangular shape to indicate a default open jumper. To open a closed jumper, just use a blade or sharp screwdriver to remove the thin connecting trace. To close an open jumper, use a soldering iron to heat up one pad, melt on solder, and use your iron's tip to bridge solder over onto the other pad. To reopen it, just remove the solder with some solder wick.