A one-step deal is a deal in which a studio hires a screenwriter to write a single draft of a screenplay, and all future work after the delivery of that draft is optional, at the discretion of the studio.
These vary from more traditional two or three step deals, where the writer is guaranteed at least one rewrite and/or polish.
On the 66th episode of Scriptnotes, John August and Craig Mazin discuss one-step deals in detail, and explain why screenwriters are generally opposed to them.