The observable universe is what, in theory, can be seen from Earth. That is light, or other signals, which has had time to reach the Earth since the beginning of universe's formation. The observable universe is a spherical volume (a ball) centered on the observer, regardless of the shape of the universe as a whole. Every place in the universe has its own observable universe, which may or may not overlap with the one centered on Earth.
How big is it?
The edge of the observable universe is expected to be 46.5 billion light years away from the Earth.