Summarize:

Even though the word "matin" means "morning, French speakers don't typically use it to say "good morning" to people. Instead, they simply say "bonjour" (bohn-zhoor). While this word technically means "good day," and is more typically used simply to say "hello," it's the common way to say "good morning" in most Francophone countries. The French, generally speaking, don't place much emphasis on the morning. For example, breakfast is a small, light meal. This may be part of the reason it's not common to greet someone specifically with "good morning." "Salut" (sah-loo) is a more casual form of "hello," similar to a "hi" or "hey" in English. If you're talking to close friends or people younger than you in the morning, this is a good way to say "good morning." The French culture is relatively formal. Even among close family members, "salut" may not be appropriate if they are older than you or have some level of authority over you. The phrase "bonne matinée" (buhn meh-tee-nay) does mean "good morning," but it's never said when you are greeting someone for the first time in the morning. Rather, you might say it as you're leaving to go on about your day. Used this way, the phrase more accurately means "have a good morning," as a wish you extend as you depart someone's company. You can think of it as similar to how people in America may say "have a nice day" as they take their leave.
Say "bonjour" to tell people good morning in most contexts. Switch to "salut" when greeting friends. Offer a "bonne matinée" as you are leaving.