A transparent glaze is made with colors that are generally more transparent in nature such as phthalo blue, quinacridone gold or dioxazine purple. Some colors are made with more “opaque” materials such as yellow oxide, white and pink. Basically, the measure of a colors transparency is measured by how much or little it covers what is beneath it.
Imagine spilling coffee on a newspaper. This is a transparent glaze. The paper would be ruined but you could still read it. If however you spilled a more opaque fluid, like a chocolate milk shake on the newspaper it would no longer remain readable. The chocolate milk shake is like an opaque glaze.
Both kinds of glazes in acrylic painting are made with a lot of water, or clear medium such as gloss or matte medium, and a smaller amount of colors.