Hey guys, if it helps I just thought I'd chime in.
The basic idea is that relativity replaces absolute space and time with relative spacetime. The replacement of absolute with relative is something fairly straight forward. The replacement of space and time with spacetime is not.
Let's take the example of not being able to move backwards in time. If I could move backwards in time I'd be able to stay where I am now, but go from 4pm to 3pm.
However it's the same with space, if I freeze time I can't go left or right or up or down, because to change your locaction in space requires motion and motion takes time.
So I can't stay in the same place and move backwards in time and I can't stay in the same time and move backwards in space.
Of course there is the difference that I can take some time and eventually move backwards in space, where as I can't take some space and move backwards in time.
This is where spacetime becomes important. Basically we live in one giant structure (or more accurately shape) called spacetime. Points in spacetime are called events, with their locations labelled by space and time, basically when and where they are. The relativity basically comes from the fact that different observers can put different space and time labels on events.
However what all observers agree on is the geometry of spacetime. Spacetime has its own geometric rules which mean that certain events can never be connected, that is you can't pass from one event to another.
One such example is travelling into your own past.
Let's say you want stay in the same place but travel to ten seconds ago, you obviously can't. For you the reason is because it's in the past.
For somebody moving past you at high speeds because they'll label events differently they'll see your "now" and your "ten seconds ago" as occuring at the same time, but at different points in space. For them the reason you can't move between these two events is because they occur at the same time and you don't have time to move between them.
These are subjective reasons, the objective reason is that the events are not connected in spacetime. Not being able to move back in time is just one subjective way of viewing the disconnectedness of two events.