Braking on a wet road and feeling a wheel lock up and skid is unsettling. In most cases the brakes are working fine - the issue is grip - and your ABS should prevent it. Here is what is really happening when brakes lock in the rain.
Wet roads mean less grip
A locked wheel is not about braking force but about traction. Wet pavement offers far less grip than dry, so the tires reach their limit and lock under braking pressure that would be fine when dry - see does tire size affect braking. Worn tires make it much worse. The brakes did their job; the tires ran out of grip.
ABS should prevent lockup
This is exactly what anti-lock brakes are for - they sense a wheel about to lock and pulse the brake to keep it rolling and steerable; see how ABS works and why ABS activates on gravel. If a wheel locks and skids in the rain and you feel NO pulsing, your ABS may be disabled - check for an ABS warning light.
When a brake fault is to blame
If one specific wheel locks easily and pulls, that can be a mechanical fault - a grabby contaminated pad or a hydraulic imbalance - see why brakes lock up. Even braking that locks all wheels on wet roads is usually grip; one wheel locking is more likely a fault.
What to do
Keep good tires with plenty of tread, brake smoothly on wet roads, and if ABS is not working get it fixed. If a single wheel locks or the car pulls, inspect that corner's pads and caliper. Quality pads and fresh hardware plus good tires give safe wet-weather stops.