VLBI uses two or more radio telescopes to observe and record the signals received from the same quasar at exactly the same time. The time difference between the arrival of the signal at each radio telescopes can then be used to calculate a very precise distance between the telescopes. How Precise? Well, VLBI can determine distance between radio telescopes to within a millimetre across an entire continent.
Radio Signals from Space
VLBI observes radio energy emitted by quasars. A quasar is an enormously bright object at the edge of our universe. The word quasar, short for "quasi-stellar radio source" was named in the 1960s when quasars were first detected. A quasar viewed with an optical telescope appears point-like and similar to a star but is really quite large and gives off energy a trillion times brighter than the Sun. Quasars are so far away, that even as bright as they are, they can't be seen without a very powerful telescope. Quasars are the most distant objects yet detected in the Universe.
How Far is Far
Distances in space are so big they're tough to imagine. Astronomers measure distances in space by how far light can travel in a year. Light travels at about 300,000 kilometers per second. I'll save you the trouble of doing the math. In one year, light will travel about 9.5 million million kilometers! The distance between us and the Sun is about 150,000,000 kilometers and it takes light a little over 8 minutes to reach us. Now lets talk some really big numbers. It would take light about 100,000 years just to cross our Galaxy from one side to the other. So, why are we talking about the speed of light if VLBI measures radio waves? Well, radio waves travel through space at the speed of light.