Just what is spacetime exactly, or more exacterer than just thinking about it?

It's a little tricky to explain, but let me try. Its simply a term that refers to a 4-dimensional space. This is purely a mathematical construct. However, Einstein discovered through the theory of relativity the universe can be described by four dimensions, three of space (x, y, z) and one of time (t). So, he cleverly called this four dimensional space space-time. These four values get combined such that each unique combination of values for these four dimensions represents a unique point in spacetime.

This is analogous to saying that in any 2 dimensional space (Cartesian coordinate system for example) each point in that space can be uniquely be described by two dimensions.

Line = 1 dimensional space P:(x)

Area = 2 dimensional space P:(x,y)

Volume = 3 dimensional space P:(x,y,z)

Spacetime = 4 dimensional space P:(x,y,z,t) <== Space time, 3 space dimensions, 1 time dimension

To clarify a bit, imagine that you live in a 3-dimensional space, a volume.

Every point in that volume can be uniquely described by an (x,y,z) point in a 3 dimensional coordinate system.

In spacetime, a fourth dimension is added, time. So, every point in space time can be uniquely described by four dimensions (x,y,z,t).

So for example,

P1: (1,2,3,5)

P2 (1,2,3,7)

Describe two points at the same physical location but separated in time. The thing is, the two points exist concurrently. Time is just another axis on a 4-dimensional coordinate system.

One way to think about it is to forget trying to envison a physical interpretation and simply use the combination of dimensions to uniquely define the location, the address if you will, of a single point in that space. There's nothing magic about 4 dimensions. You could create a 6 dimensional space for example.

P: (a,b,c,d,e,f)

There wouldn't be a physical interpreation of this space, but there would be a mathematical one. Each combination of values for a,b,c,d,e,f represents a unique point in that 6-dimensional space.

Hope this helps,

-Guru