What is meant by the fourth dimension?
When we look at our world, we describe objects as three dimensional.
We can see that an object has certain length, width and height.
The same for a coordinate system. We can describe any point in space with three numbers (the simplest is the x,y and z coordinates).
However things in the world are not static. Objects move from one place to another, materials can change phase (water to ice for example).
Hence when we describe a physical process, it is not enough to describe its three dimensional aspect, but also the *time ...