Monolith vs Microservice, Please ELI5

I dont really understand the difference between these two patterns.

Let's say we have a node application with a set of routes for looking at product info and buying products

So something like

app = express(); app.install(buying_routes); app.install(info_routes); app.listen(); 

in our server

Is a microservice architecture simply placing these routes in their own server?

Also people say the microservices can interact by calling each other's APIs. Couldn't you do that in the monolith too? the routes in the buying_routes
could call the routes in the info_routes.

I think I'm missing something.