A paint color is said to be opaque when it hides what's underneath it, when you can't see any or much of what's beneath the color. Some pigments are extremely opaque, such as titanium white and cadmium red; other pigments are semi-opaque, such as zinc white. Opaque does not mean white.
The label of the paint tube should have an indication of whether that color is opaque or not. (Cheaper brands sometimes lack this information.)
The opposite of opaque is transparent.