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, which can be roughly spherical, or a flatter circle, or can have a longer dimension, forming a straight line between 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 (SFR), which is reasonable if the visible bar-appearance is from the bright light of many early type stars in the region.