This is a type of publishing deal. In an admin deal, you, the songwriter, keep 100% ownership of your copyright and give away 10-25% of your publisher’s share in the form of an administrative fee. The term of an admin deal is usually between 1 and 3 years. Publishing administrators do not own or control any percentage of your copyright at any point during or after the term of the agreement.
Administration agreements ordinarily do not include any creative services, and focus solely on administrative duties such as properly registering your songs with collection societies around the world as well as collecting royalties on your behalf. Admin deals can occasionally include an advance, which must be recouped in the same way as in a co-publishing deal.
Publishing administrators like Songtrust have relationships with performance and mechanical collection societies around the world that make royalty collection much simpler than it would be for a single songwriter to do on their own, and this leaves you time to focus on writing as well as to work on pushing your songs creatively. You can learn more about the terms of Songtrust's administration agreement here.