If I am talking about the same lens your 18-135 has a widest aperture of f3.5. The 50mm has a widest aperture of f1.8. Basically at f1.8 you can let in more light through the lens, this allows for a faster shutter speed with two results. Firstly you can shoot in low-light situations, often without flash (indoors) and secondly allows for sharper images.
That is a layman's explanation. If you want the techies explanation then you'll have to wait for someone else
