Any object can be set to be semi or fully transparent:

The illustration on the left is made up of two objects. The left object is red at 0% transparency i.e. no transparency so it shows the full red colour. The right object is the same red at 50% transparency.

In this illustration, three objects are set at 50% transparency. Each object is intersecting – objects underneath a semi-transparent object will show through.

Objects can be made transparent in different ways. One way is to select the object and use the slide bar in the Main Toolbar to set the level of transparency: