To place something is to put it somewhere, and to replace something can mean to put it back where it goes, as in, "After we finished our game of checkers, we replaced the pieces so that other folks could play."
(Source) Here's another common meaning of the word "replace." When something is broken, or missing, or all used up, or you just don't like it anymore, you might replace it: get a new one, or get a different one, to use instead. When your light bulbs burn out, it's time to replace them: put in new ones in their place. If your lamp breaks, and it can't be fixed, it's time to replace it: get a different one.
(Source) Has your backpack fallen apart? It's time to replace it! You'll need to buy a new one as a replacement. Did you borrow a book from a friend, then accidentally lose it? It's time to replace it! You'll need to buy a replacement: a new copy of the same book to give back to your friend.
In some situations, you can replace people, too. If you play on your local basketball team, but then you have to move to a different state, then your coach will replace you: get another person to join the team in your place. If a worker quits her job, her boss will probably have to replace her: get another person to do her job after she's gone. And if a worker's job is easy enough to be done by a robot, then the robot might replace the worker.
Things that you can easily buy (if you have to money for them) are replaceable: they can be replaced. If a windstorm destroys your fence, well, look on the bright side: your fence is replaceable, so you can get a new one. But things, people, and pets that can't be replaced—because they're unique, one of a kind, or really special to you—those are irreplaceable. If they break, or get lost, or die, they're gone forever, and nothing will ever replace them. A good friend is irreplaceable, and a beloved pet is irreplaceable. And, of course, your family members are irreplaceable.