Exactly! The wheels on the left and right of a car are generally able to rotate at different speeds, this is helpful when going around corners, as one wheel needs to spin faster than the other (more distance in the same time). This is controlled by a differential. There are a couple different types of differentials, but the basic idea is that when power is sent to the axle/wheels, one is side is allowed to slip. This is bad when only one side of your axle has traction (ex wheel in the air) because the power will take the path of least resistance and spin the wheel without traction and the other wheel will slip. With both wheels in the air - no power was available to push the car forward. When they mention locking the diffs in the video, they’re talking about enabling a feature which prevents the differentials from slipping so that power is sent equally to the left and right wheels on an axle which would push the car forward with a drive wheel in the air.