A bar is a feature of some spiral galaxies which are said to be barred, i.e., barred spiral galaxies. Spiral galaxies tend to have a region with a lot of stars in the center (a bulge), which can be roughly spherical, or a flatter shape, or can have a longer dimension, forming a straight arm joining the inner ends of the spiral arms, in which case it is known as a bar. The Milky Way is now presumed to have a bar. The abbreviation SB in the galaxy classification is used for barred spiral galaxies as opposed to S without the "B" for a spiral galaxy without a bar.
The dynamics the produce this morphology is of interest. Bars are thought to contain star-forming regions, which is reasonable if the visible bar-appearance is from the bright light of many early type stars in the region.
The term bar is also used in a totally unrelated manner, for a unit of pressure, intended to be close tot he pressure of Earth atmosphere.