A principal characteristic of a lens is its maximum aperture – the maximum size of the hole through which all light must pass. The wider the aperture opens, the "faster" the lens, because in still photography, a wide aperture allows the use of faster shutter speeds to minimize motion blur (although this is not something most videographers would dApertureo).
Some zoom lenses employ a variable aperture mechanism that can use its widest aperture only within a certain focal range. As a rule of thumb, the faster a lens is, the more expensive it will be (assuming that other factors, such as optical quality, are equal). In photography, a fast lens is great for taking pictures in low light, but when shooting video, a very wide aperture impedes the ability to pull accurate focus due to the extremely shallow depth-of-field it creates. This is more of an issue with full frame cameras than with a cropped sensor. A 2.8 aperture is typically as wide as the shooter will want to go (on an APS-C camera) in order to control depth-of-field. That means a 1.4 lens won’t be used at its maximum aperture unless absolutely necessary.