A bar is, simply put, a tavern or public house located in a spaceport. However, in the context of Nova, a bar is a social center and hiring hall for all manner of spacefarers and those who do business with them. Certain types of employment (generally involving illegal activity or requiring confidentiality) are only available in bars. Bars are also the place to hire escorts or to do a little gambling.
For an excellent example of a spaceport bar, see the "Cantina" scene in the original Star Wars film (Episode IV, A New Hope).