If you turn the steering wheel and the car doesn’t want to turn, that’s understeer. If you find this happening to you, it’s probably because of a heavy right foot. Understeer generally occurs either (1) during hard braking or (2) during hard acceleration. If you’re trying to mix turning with accelerating (positive or negative), you have to modulate your right foot and only apply as much brake/throttle as allowable by the remaining traction.
Braking understeer generally occurs on corner entrances. The typical scenario is the driver brakes too late and too hard, and there’s no grip left for turning. Tires have only so much grip, and you can use that grip for braking, turning, or some mixture (the relationship here is often thought of as linear, but it’s not strictly the case, especially when skidding). If you brake really hard, you can’t turn, no matter how much you wind the steering wheel.
If you find yourself in this situation, you have to release some brake pedal pressure to restore your ability to turn. If you have to think about what to do, it’s already too late. The sound of skidding tires and the lightening of the wheel should trigger your muscle memory to stop braking. If you don’t have that mental programming, the cheapest way to acquire it is through sim racing (see the How To link above).
Power understeer generally occurs on corner exits. Pressing the throttle shifts the weight to the rear of the car, which reduces the load and traction on the front tires. If they loose too much traction, they may slip, causing the car to drift off to the outside of the turn. Front-wheel-drive cars with a lot of power can spin their front tires for an even more dramatic loss of traction.
If you watch that whole clip, you’ll see both kinds of understeer.