'Factorise' essentially means put into a simpler form, in this case with two sets of linear equations which would multiply out to give the quadratic equation shown above.
What we are looking for to begin with is two numbers that add together to give 7, and multiply to give 10.We reach the numbers 5 and 2 in this example. We then simply re-arrange the quadratic into the form ' (x+a)(x+b)', where a and b are the two numbers we have found that multiply and add respectively to give 10 and 7.
Therefore, factorised, x^2+7x+10 is (x+5)(x+2)