It flows through the universe. Like a river full of water, it is conceivable to tunnel through the river to connect one point on the river with another. It's possible to put people in through the tunnel. Time travel is not now a fantasy. He explains the background to his discovery.
Time is what our culture measures with a watch.
For Einstein, Everything on the table exists in space and time - or 4D spacetime as he called it. Gravity is the curvature of spacetime around matter. Novikov describes time as being like a river. You can, in principle, tunnel back in spacetime and rejoin the river upstream of where you rafted in. The suicide paradox, for example is presented as the reason time travel can never exist.