EJBroeders did an excellent job of explaining this for smart adults! I'm going to try my hand at an analogy that hews to the "five years old" side of things, just in case a more generic tale is helpful :)
Imagine you have one best friend with whom you do everything! You read together, ride bikes together, and talk together. One day, your best friend is sick, so you can't read with anybody, or ride bikes with anybody, or talk to anybody, because you only have the one friend.
You decide that you'll make three new friends instead: one who loves to read, one who loves to ride bikes, and one who likes to sit around and talk. One day, your friend who loves to ride bikes falls off her bike and can't ride bikes that day. Fortunately, you still have somebody to read with and somebody to talk to!
Your one friend with whom you do everything is a "majestic monolith" - it's great to have a relationship like this, but it's hard to cultivate and potentially catastrophic if something goes wrong with it. Your group of three friends are microservices - you're spreading the risk around by making friends this way, at the possible expense of more difficult communication, and missing out on a deeper connection.
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