Spacetime refers to whatever external reality underlies our collective experiences of the space between things and the time between events. It is basically a mathematical model that combines space and time into a single interwoven continuum.
Space consists of three dimensions, while time consists of one dimension (the 4th dimension). By combining space and time together, physicists have significantly simplified a large number of physical theories, as well as described in a more uniform way the workings of the universe at both the supergalactic and subatomic levels.
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Another theory stemming from spacetime is that time itself is an illusion. Yes, it certainly feels as though time flows from moment-to-moment, but in reality you are only aware of this present moment.
Einstein theorized that time is actually relative and flexible. In fact, he even mentioned that the dividing line between the past, present and future is an illusion. And that essentially would make reality timeless. Here is a video that explores this idea in further detail.