Read Special Relativity by Albert Einstein.
Seriously, it's not that hard and there's hardly any maths in it.

(Avoid the General Relativity fro just now though, lol. That DOES have a lot of maths..)
[link to www.amazon.com]

The point is that 3-d space and time are related - we can't just move anywhere anyhow anytime... So time becomes a dimension we need to consider.