Opacity controls how see-through an item appears. It is like looking through a clear curtain: at one end you cannot see through it, and at the other end it is completely transparent.
A designer might lower the opacity of a background image so words placed over it are easier to read. A faded control can also suggest it is temporarily unavailable.
Do not rely on faintness alone to communicate meaning. Text that is too transparent may be hard to read, and people should still be able to tell whether a control is disabled or simply decorative.