BMS
Battery Management System
The computer that looks after the battery: it watches each cell's voltage, temperature and balance, and it's the one that reports the SoH you see in the car.
The BMS is the battery's brain. It keeps any cell from exceeding voltage or temperature, spreads charge evenly (balancing), and keeps the running tally of the pack's health.
The SoH your car shows is computed by the BMS, and it's the manufacturer's own estimate — sometimes smoothed or conservative, and very different between brands. That's why an independent reading like SOHpro's adds value: a second pair of eyes on the same pack.