The term baseline can mean two different things in the astronomical field. In general astronomy, a baseline is usually the reference point when two or more values are being measured. It is extremely helpful when interpreting astronomical data because it can ignore any variation which can muddle the true measurements of something, such as spectral line data.
On the other hand, in radio astronomy, while looking at a interferometer, a given distance between two aerials is called a baseline. This is important to the field of interferometry, which attempts to learn more about the properties of waves through superimposition. The instrument for superimposition is called the astronomical interferometer. Interferometers are also used in other astronomical fields, such as optical astronomy and infrared astronomy.
A technique called A Very Long Baseline Interferometry (VLBI) is particularly useful to radio astronomy, because it can allow multiple simultaneous observations to be combined. This can be considered an alternative to having a telescope which has a size equivalent to several telescopes which have the maximum separation between them. This can provide images though the telescopes may be separated by thousands of kilometers, without any loss of quality.
The VLBI is mostly used for spacecraft tracking and imaging distance sources from the radio. Because the distance is no longer an issue, the VLBI has made more astronomical discoveries possible.