Good question. You might even say, how do you understand the concept of time? There are many ways to consider it, and the model you choose depends on what you're trying to accomplish.

However, here's a simple model that works. Think of time like a distance. However, both distance and time are measures of a difference between two states (an initial state, where it begins, and a final state, where it ends).

For example, you can consider two points in space (different places), and look at the distance between them (it is 2 miles between my home and work), or you can consider two points in time (different events), and look at the time that separates them (it has been 6 hours since I left home).

Their similarity isn't totally by chance, and you may have heard of the theory of relativity, which showed that space and time are related in strange and unexpected ways!

Wow, what a deep question!I would be lying if I told you that I could give you a definitive answer: this is a question that physicists have been struggling to understand for centuries, and we're still far from any good answer. We do have some ideas, however; here's a common one:

You can think of time in terms of entropy. You may have heard the term "entropy" before, but if you haven't, it's basically a measure of the disorder of a system. So, for example, if all the clothes in your room are neatly folded and organized in your closet, then your room has a lower entropy than if you took all your clothes and threw them at random around the room. Now, the second law of thermodynamics says that the entropy of the universe is always non-decreasing; that is, the universe always gets "messier," or at least stays the same, but never gets "neater" (you may experience this with the clothes in your room, as well!). You can think of "time" as describing this continuous change from a more ordered to a less ordered state of the universe. To give another concrete example, if you drop an egg on the floor and it cracks, you increase its entropy (the whole egg has less entropy than the broken one). But since entropy cannot decrease, we can't reverse the flow of time to make a cracked egg whole again. This concept is referred to as the "arrow of time," and refers to why time seems to only flow in one direction.

Whoever answers that one is going to get the Nobel Prize.

Time is a dimension, but it has some properties that make it different from the other three dimensions (what we call space) that we are familiar with. What makes time different and why are some of the big unanswered questions in physics.