A logline distills a story to a single sentence built to hook a reader or buyer: a protagonist, their goal, the central conflict, and ideally a twist of irony or stakes. It overlaps with premise but is pitched outward — it's a selling tool as much as a focusing one.
Writers also use loglines internally: if you can't write one, you may not yet know what your story is about.
Example
'A burned-out hostage negotiator must talk down the one criminal she can't read — her own estranged son.'