Basically, you have an aliquot (fixed amount) of base sitting in a clean beaker.
What you want to do is to react it with acid, to neutralise the base completely with sulfuric acid, so that the pH jumps from basic to acidic. The methyl orange indicator will be what allows you to experimentally see this. The point at which the indicator changes colour, is called the "end point." The "equivalence point" is when the number of moles of the base is equal to the number of moles of the acid (accounting for stoichiometry), so that this where a steep change in pH occurs. In experiments, we assume the end point is the equivalence point (since it roughly is -- the end point comes slightly after the equivalence point usually), and hence we can use the number of moles of the substance with a known concentration (in this case the base), and match it up with the number of moles of the substance with an unknown concentration (acid).
Basically, you know the number of moles of sulfuric acid you delivered, because you titrated it until you reached the end point (roughly the equivalence point), and hence you can substitute in the number of moles of the base. Then you use your titre (a volume) to divide through and get the concentration of the acid.