ESCs (or Electronic Speed Controllers) take the signals from the flight controller and current from the battery to tell the motors what to do. Traditionally these have been 4 separate pieces secured to each arm of the quad but there's a growing trend to have a 4-in-1 ESC that stacks below the flight controller. A 4-in-1 ESC is better protected from prop strikes but if one ESC blows for some reason, all 4 need to be replaced.
We stock both KISS and BLHeli (including BLHeli_S and BLHeli_32) ESCs in a range of outputs including 20Amp, 25Amp and 30Amp. Bigger motors with more aggressive props need a higher amp rating.
Newer ESCs support newer features like the digital protocol DSHOT, telemetry and more.
If you want to know more about ESCs, we have a longer article here as part of our Intro to FPV series.