Early Day Motions (universally known in Parliament as 'EDMs') are best thought of as petitions put forward by MPs on specific subjects. They are printed in the official report and other MPs are invited to add their names in support.
Technically, EDMs call for the topic to be debated at the earliest time but the procedure is now little more than a means of drawing attention to a subject. They are often used by back-benchers in the governing party to express concerns at a government policy. The only real chance an EDM has of being debated is if the Leader of the Opposition sets it down.