It means that every time they write something in the contract in the singular (for example "He" or "Him" it includes a possibility that it refers to the plural (which would be "they" or "them") unless that is impossible for that situation.This saves time when you write a document so you don't need to write he/she/they every time you write something.

This sounds like a contract. You would probably need to have C1 (advanced) level to make sense of contractual clauses like this. It is a standard clause in many contracts which clarifies that the parties should not normally interpret the contract differently if they see a plural noun as opposed to a singular one. If there were ever a legal disagreement which related to the interpretation of this kind of clause, I would be very surprised! Does this make sense to you?

in this agreement, the following expressions shall have the folloeing meanings and the singular shall include the plural, save where the context makes that impossible:

Can you give us some context for this sentence? :)