In photography, a "walk around lens" is exactly what it sounds like. It is an all around decent lens for a variety of subjects and situations. Typically it is a zoom lens with an adjustable focal length. Zoomed out the lens has a short focal length, which is typical for landscape and scenery photography. Zooming in increases the focal length, changing the perspective – giving the photographer a shallower depth of field. This is often seen in portrait and isolation of form photography. It's great to be able to grab a light weight rig and run out of the house with your favorite lens attached. Unfortunately no lens is perfect for every situation so it's a choice between specialization and convenience.