A logline is a one-sentence summary of your screenplay.
The term "logline" was first used in old Hollywood. The big studios would own hundreds of scripts, and the studio head would keep a log book that recorded concise summaries (or "loglines") that described each script in the studio's possession.
The fundamental unit of structure for your screenplay is the logline. It should both describe the plot and grab the reader's attention.
A well-crafted logline will help you to stay focused during the writing process and communicate your idea to agents, producers and potential collaborators.