For instance, most people are familiar with polynomials. Take a polynomial in x (with integer coefficients) and substitute x for 5t everywhere. So for instance, 2x^3 - 8x + 3 becomes 250t^3 - 40t + 3.
The Rees algebra Z[5t] (here Z is the integers) is then just the set of all polynomials you can get this way.
If Wikipedia introduced it like this, I don't think most people would have a problem understanding.
The concept is not (at least not always) actually that complicated, like others are implying. It's our communication that is lacking.