How to understand "against something"?

Solution 1:

This usage can be found in Macmillan sense 6:

against: used for comparing one thing or amount with another

• This year’s profits amounted to only $35 million, against $52 million last year.

Collins Cobuild (sense 11) gives a clearer example:

If something is measured or valued against something else, it is measured or valued by comparing it with the other thing.

• Our policies have to be judged against a clear test: will it improve the standard of education?

Check your answers against the answer-sheet (= Check your answers against those on the answer-sheet.) [HMHCO]