A pentaprism is what allows the user of an SLR to see directly through the lens (TTL) in the viewfinder.
On its own, a camera lens renders an image that is both vertically inverted and laterally reversed. While the mirror of a reflex camera reinverts it vertically, the image is left laterally reversed.
A regular pentaprism is a five-sided optical glass element that is used to deviate a beam of light by 90 degrees. The beam reflects inside the prism twice, allowing the transmission of an image through a right angle without reversing it.
SLR cameras therefore require a variant of a pentaprism which will not only deviate light by 90 degrees but also reverse it laterally (left-right). The roof pentaprism does this by replacing one of the flat reflective surfaces with a roof section, two additional surfaces angled toward each other and meeting at 90 degrees. Combined with the mirror this allows light from the lens to be reflected into an eye-level viewfinder with the picture appearing the normal way to the photographer (illustrated on the following page).
This avoids the problem suffered by the twin-lens-reflex viewfinder system, which has the image laterally reversed, making framing difficult and following a moving subject confusing. The signature bump on top of an SLR is where the pentaprism is housed.