Hi Coldplay,
A matrix is an array of numbers.
It is used a lot to solve difficult simultaneous equations where there is more than 2 unknowns.
If there are 2 unknowns you need 2 equations
If there are 3 unknowns you need 3 equations.
The coefficients of the letters (pronumerals) are the numbers that go into the array.
Have a look at this introduction
http://www.mathsisfun.com/algebra/matrix-introduction.html