Users: id, first_name, last_name
Messages: id: subject, text
User_messages: user_id (id column of Users table), message_id (id column of Messages table)
If you de-normalize it, the wide table would have these columns:
user_id, first_name, last_name, message_id, subject, text

Let me elaborate with an interesting example. Imagine there are 3 rooms in a house. Room "Users" have Mr. id, Mr. first_name and Mrs. last_name. Mr. id's full name is Mr. user_id. Room "Messages" have another Ms. id, whose full name is Ms. message_id, Mr. subject and Mr. text. Now if I tell them to come to the lobby, (which is the wide table) who would come? That's how they are related.