A KiCad symbol used to tell ERC "this net is driven from outside the schematic." Sits on any net where only power_in pins exist and there's no power_out pin to satisfy the driven-rail check.
How it looks
Drawing: the CORE3588 schematic uses no PWR_FLAG symbols
The flag tells ERC "power really does arrive on this net".
How it works
ERC checks that every net feeding a power input pin is driven by some power output.
Power that enters through a connector has no "power output" in the schematic, so ERC complains that the net is undriven.
A PWR_FLAG symbol on that net says "driven from outside the drawing". It adds nothing to the board; it only satisfies the check.
Where it fits
A tool for clean ERC; like the no-connect X, it records an intention.