A paint color is said to be transparent when you can see through it. How much you can see through it depends on what pigment was used in that particular color (or hue), as well as how thickly or thinly it is applied.
Transparent does not mean clear or colorless. Think of sunglasses, how the lenses have a color yet you can see through them, that's what is meant by a transparent color.
The label of the paint tube should have an indication of whether that color is transparent or not. (Cheaper brands sometimes lack this information.)
The opposite of transparent is opaque.