"Accepting" something means saying "Yes" or "OK" in respnse. You can:

accept an invitation

accept a challenge

accept an idea

accept an offer

An "offer" is something that a person tries to give you. Here are some common "offers" that people make:

offer to help

offer (someone) a job

offer a salary

So "accepting an offer" means taking something that someone tries to give you, like a job.