Factorising is a way of 'converting' an equation into a form with brackets in order to make it more useful and easier to manipulate. It is the reverse of expanding an equation.
For example:
To factorise the quadratic x2+5x+6, you need to look for a pair of numbers that multiply to give 6 and at the same time, add up to 5.
In this case, this is 3 and 2.
This means we can factorise and write the equation x2+5x+6 as (x+2)(x+3).