In Brane cosmology, a four-dimensional universe is restricted to just a membrane inside a higher-dimensional space, called the Bulk.
The Bulk Beings are sentient beings in the Interstellar Universe able to perceive these higher dimensions, and hence are able to see time just as another physical dimension. This allows them to create wormholes and higher-dimensional tesseracts.
The tesseract shown in the film was able to manipulate time itself as a physical dimension, thus being capable of showing and interacting with past events. It is alluded to in the film that these "bulk beings" may in fact be humans existing millennia in the future that possess unfathomably advanced technology and have evolved to the point of existing outside of the four (known) dimensions of time and space.