Light cannot pass through opaque objects. This means that you cannot see through them at all.

In this first image, there is a piece of thick cardboard held up against a light background, you can see that no light passes through the card hence it is opaque.

Below is another example of an opaque window frame, no light gets through the window frame as it is made of wood.

Note that the window frame is opaque, the glass in the window is not.