Every agent works a little bit differently, but I’ll tell you how the agents I’ve known work:
Usually an agent expresses interest in one or more of your manuscripts.
Then you need to sign a contract with an agent. This is not a contract to publish a book, but it is a contract that says the agent will represent you in the publishing industry. This contract doesn’t guarantee the sale of your book to a publisher, but it provides legal guidelines to your relationship with your agent. Such as which books she’ll represent for you and how the money will be handled when you get a book contract, etc.
Then you submit more manuscripts. I try to send brand new manuscripts to my agent. (They rarely want to see a manuscript that you’ve already submitted to publishers because if the publishers have rejected it already, how can the agent expect to sell it?)
The agent provides feedback.
They might say it’s just not something they feel is quite right for the market
Or they might say they’re interested in representing it
At this point, some agents will help you edit it A LOT because they know what the editors they work with like.
Some agents don’t edit too much because they like to keep the author’s voice intact at this stage and wait for the actual editor to give more input.
Then it’s our job to revise the manuscript in a timely manner per their requests. We resubmit it and rework it until it’s what the agent wants to see.