Time. The great existential mystery,what is it?

Time is not just a fourth spatial direction, like many people like to think. Even in physics, the time direction is considered special by putting a minus sign in front of the time term in the Lorentzian norm.

Time can be thought of using the units which we measure, but special relativity tells us that these are not invariant to all reference frames. A better way to think of time, according to me anyway, is the "well ordering" of events. Time is the way in which we say something happened before or after something else. Even this is tenuous: special relativity also says we will not always agree on the order in which things happen. However, we can postulate that there is some "preferred" reference frame like Lorentz or Poincare postulated, and from this we can take a more absolute perspective of time. This is the way I think of time.

Many physicists do not like this assumption. A major point of Einstein's special relativity paper was to remove that preferred reference frame. A different view of time that matches up with this uses the flow of entropy. We move in time with regards to the direction in which entropy increases.

Along with Space, Matter, and Energy, it's a creation of the Quantum World, to facilitate our macro-holographic Virtual Reality. Alternately, it's an internal limitation on the processing speed of the universal quantum computer.

one of the 4 essential dimensions necessary to define a physical event.

All events require time and space.

time is just as real as length, width, and depth and just as necessary.