if he likes it. Typically this happens when you send a query to a publication or book publisher asking if they are interested in looking at your submission. If they agree to look at it, the letter or email indicating that will say they will look at it on speculation. That indicates they are making no promises to buy it—which they will do only if they like it and they find a place for it in their publication. Most freelancers will continue the write on speculation until they are well enough known by the editor that the editor feels comfortable enough with that writer's abilities to actually make definite assignments.
Some more experienced writers make it a policy to never write on speculation and work only with a firm assignment.