Optical activity is the ability of a chiral substance to rotate the plane of polarized light. It is measured using a device called a polarimeter.

What is polarized light? 
Normal light consists of electromagnetic waves that vibrate in all directions.

When light passes through a polarizer (Nicol's prism) the electromagnetic waves vibrate in a plane. This oscillation plane coincides with the propagation plane of the wave. Enantiomers and Polarized Light
When polarized light passes through a cuvette containing a chiral substance, a rotation occurs in the plane of polarization.
Chiral substances rotate polarized light and are said to be optically active (show optical activity).

Those substances that do not produce rotation in polarized light are optically inactive. Right-handed and left-handed
When an optically active compound rotates in polarized light clockwise, it is said to be right-handed and is represented by (+).

Substances that rotate light counterclockwise are left-handed and are represented by (-). The enantiomers rotate polarized light the same angle but in opposite directions. This angle can be measured using a polarimeter.