Many camera kits come with a 18-55mm zoom lens, which is a versatile lens under common shooting conditions. But if you want to take portraits with nice bokeh, or shoot in low-light conditions, these kit lenses usually aren’t optimal. Most of these lenses have a maximum aperture of f-4/5.6 (depending on the focal length you’re shooting at) and it can be a bit tough to achieve these effects with that aperture.
Many photographers swear by an f-1.8, 50mm lens. Some even say this is their main shooting lens and they barely use other lenses.