How’s this analogy:
Your Facebook Profile is like your home. You “hang” your pictures on your walls, and leave the photo albums out for (random) people to look through. You can choose who you let in, and you can even have conversations with specific people via “lists“. Lists are a very under used feature. But you also can choose if the public can see what you share too. Sort of like sitting on your front porch.
Messenger is like using the phone, only it’s usually more like a instant telegram. You can have a one on one chat or invite several people and have a group chat… usually for planning things. But can also be to discuss topics of interest. Although many people are now using Facebook’s phone and video chat.
Next are Pages. Pages are like store fronts. Anyone can check out what’s going on and participate. This is one way Facebook makes money. Page owners buy ads to bring people into their “stores“.
Finally, there are Groups. Groups are like rooms. Sometimes, they are public (the door is open) and anyone can come in. Sometimes you can see who is in the room, like a room with a window. But you can enter only by invitation or by asking for approval to join (you have to knock). Some are secret rooms. You don’t know they are there. You can only see it and participate if you have been invited.
This is a simplified description. There are many more features that Facebook offers, such as Events, that I have not covered here. But if people view their online presence as they view their real world presence, they will understand how aligned both worlds are.