Current-sense resistor
also known as: Shunt resistor
A deliberately small, known resistance placed in a current path so the voltage across it reports the current: I = V/R. Milliohms to single ohms, chosen to drop enough to measure but not enough to matter.
In practice
It's how chargers regulate (the TP4056's PROG current is 'whatever drops 0.6 V here'), how fuel gauges count coulombs, how motor drivers detect stall. Power rating matters: P = I²R lands on a part chosen for smallness.