When we talk about an expanding universe in the framework of general relativity, we mean an expansion in a higher dimension. Like the concept of curved space, we can only visualise such a concept with the help of a two-dimensional analogy. Imagine a balloon with a large number of dots on its surface. The balloon represents the universe, its two-dimensional curved surface representing the three-dimensional curved space, and the dots on the surface, the galaxies in that space.
When the balloon is blown up, all the distances between the dots increase. Whichever dot you choose to sit on, all the other dots will move away from you. The universe expands in the same way: whichever galaxy an observer happens to be in, the other galaxies will move away from him.
Using the distance from the galaxy and its recession velocity, Hubble’s law – one can calculate the starting point of expression – in other words – age of the universe.