A log line is a one- or two-sentence description of a movie. The purpose of the log line is to present the story in a way that is concise and will grab someone’s attention. To do that, the writer must boil the story down and present it in a dramatic way.  Here are a few examples of log lines (from inktip.com):

Independence Day: Aliens try to invade earth on Independence Day.

Liar, Liar: An attorney, because of a birthday wish, can’t tell any lies for 24 hours.

Dead Calm: A married couple, trying to recover from the death of their only child, are terrorized at sea by a handsome maniac.

The Hunt for Red October: A Soviet submarine captain uses Russia’s ultimate underwater weapon as a means to defect to the west.

The Last Boy Scout: A private detective must team up with an ex-football star to catch the killer of a topless dancer.

Writing exercise: Write 4-5 log lines. Then test them on your friends. Find out which stories grab their interest.