A dimension is something we use to specify the location of something in space (and possibly in time). Let's concentrate on dimensions in space for now.
A zero dimensional space is just a point. Anything in that space needs nothing to specify its position, since anything in the space is in the same place.
A one dimensional space is like a line. To specify where something is on a line you need one number (the distance along the line from some arbitrary starting point). One number implies one dimension.
A two dimensional space is like a plane. You need two numbers to specify distances in two different directions. For example, to locate something on the floor of a room, you need to say how far it is from two perpendicular walls.
A three dimensional space has height as well as width and breadth. To locate a particular air particle in your room, you need three numbers to measure distances outwards from two walls and upwards from the floor.
All these numbers are <i>minimums</i>. In a two dimensional space, you might have lots of different ways of specifying position, but each unique specification needs at least two numbers.
We can extend these ideas to four and more dimensions. In a six dimensional space, we need six numbers to specify the location of something.
Our universe is spatially three-dimensional as we perceive it. We can, in principle, locate any point in space with just three numbers.
