A multiple star is a group of
three or more stars that are
so close to eachother that
to the unaided eye they appear
to be a single star.
They are held together my their
gravity, two of them forming
a binary star, while the
third star (or sub-group)
orbits around the opponents'
center of gravity.
If the luminosity or color of the stars
in the group vary significantly, they
can appear to us as a variable star.
These fluctuations in apparent
brightness can be used to estimate the
size and luminosity of the
individual stars in the system.
Multiple and binary stars are very
common. Well known examples include
Castor (in the constellation
Gemini), Mizar (in Ursa Major),
Polaris or the Pole Star (in Ursa Minor),
and our nearest neighbor in
the Milky Way, Alpha Centauri.
Trying to separate close pairs of stars
is a good way to test the optics
in your telescope.