The unification of relativity and quantum mechanics by Murph allows humans back on Earth to fully understand gravity and master it. This allows them to raise big stations up in space by lowering gravity (NASA's facility was one such station being constructed), just like in the original script where they build a device to cancel Earth's gravity and send humanity in space effortlessly.
The closer we are to the event horizon of a black hole the slower time passes, until the event horizon where time stops. So it seems a good guess that beyond the event horizon time might actually start running backwards, and that seems to be the take of Kip Thorne and Nolan in Interstellar: inside the black hole, the humans from the future build a tesseract for Cooper in the present to send a message to his daughter in the past, as if inside the black hole the future happens before the present which happens before the past.
Tars calls these humans from the future beings who live in the bulk. The bulk is a term used in recent physical theories related to string theory (see the wikipedia article on brane cosmology), and it refers to a hypothetical higher-dimensional space encompassing our conventional 4-dimensional space (3 spatial dimensions and 1 time dimension). Tars calls them 5-dimensional beings, so in Interstellar the bulk consists of one additional dimension. According to the theories only gravity would be able to travel between the bulk and our conventional dimensions. In Interstellar, Cooper communicates with Murph through gravity (that is how he moves the books, interacts with the watch and makes the dust pattern in Murph's room which Cooper interprets in the beginning as a gravitational anomaly).
In the bulk of Interstellar love is a physical, tangible reality, not just the unquantifiable emotion that it is in our conventional dimensions. Brand was right. The beings of the bulk (the humans from the future) hence physically perceive the love that links Cooper and Murph in her bedroom, through space and time, and the tesseract they have built enables Cooper to interact with the other end of that link. Apparently these beings are not able to directly interact gravitationally with our conventional dimensions (because if they did then why wouldn't they communicate directly with Murph themselves?), which then leaves the question who built the wormhole? If it is them then I do not yet understand why they couldn't help Murph directly the way Cooper did using gravity.
edit: actually I think I just understood, they do state that love is the one thing that can transcend time and space, so the beings of the bulk cannot use the love that links Cooper and her daughter to communicate gravitationally with Murph in the past, but Cooper can! And that's what they help him doing. Still the question remains, is it the beings of the bulk who built the wormhole?
Cooper is sent back near Saturn through a wormhole once the beings of the bulk close down the tesseract. Cooper sees Brand in the wormhole, at the time when she was in the Endurance going through the wormhole for the first time. Cooper is the gravitational being she interacted with at that point. I suppose that the wormhole Cooper went through after the tesseract connected to the first wormhole at some point, but it is not clear to me why Cooper saw her inside the wormhole that she had went through 74 years earlier or so.
Cooper leaves at the end to reunite with Brand, who still believes she might be the last human alive, as she thinks plan A has failed and that Cooper died in the black hole. Until he reaches her she is on her own, trying to fulfill plan B and save humanity, by growing the fertilized eggs they had brought with them on the mission.
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