A transparent material shows a clear view through to whatever is behind it; a translucent material scatters light so the view behind is blurred; an opaque material blocks light entirely, so its own surface colour is all that's visible.
What separates the three is how far light travels into the material before it comes back out. A transparent material lets light pass with almost no scattering, so the view through it stays sharp. A translucent one scatters light as it passes, so light gets through but the image does not. An opaque one absorbs or reflects everything at the surface, so only its own colour is visible.
Matters most for glass, sheer fabric, coloured varnish and any glaze, where the final appearance is a combination of the material and whatever sits behind it. Matters least for ordinary opaque wall paint at full coverage, where the substrate has been eliminated from the result — though it matters again at one thin coat, which is effectively translucent.
Choosing a translucent finish — a wood stain, a glaze, a sheer curtain — from a sample viewed against a white background, then applying it over something darker. The result is a different colour, because the background was always part of what was being seen.
Practical advice
Coloured glass, sheer curtains and coloured varnish all rely on what's layered behind them, so the same nominal colour can look quite different over a light versus a dark background.
Always confirm with a physical sample in the actual finish before committing to a large surface — see methodology.