A "Courtesy Enter" is sending of an IM message sooner than the entire intended message is actually written, in case of writing a larger wall of text or poor writing skills. The sender thus relieves the other party of waiting too long to receive the message, allowing them to remain focused on the correspondence by keeping a steady flow of new information.
You're chatting. The other party is typing something for ages.
You: Courtesy Enter?
Result: You get a part of the whole message, your friend continues to type the rest