When in Override mode, an animation layer overrides the animation of layers that share the same properties and precedes it in the stack.

For example: AnimLayer1 and AnimLayer2 are both in Override mode. A sphere’s Translation value along the X-axis is 10 on AnimLayer1, and 15 on AnimLayer2. In the result animation the sphere will have a Translation value of 15 for the X-axis.

You can modify the opacity of Override layers by turning on the Passthrough option and animating the Weight value. See Override-Passthrough mode, Setting the animation layer modes, and Keyframing the weight of animation layers.