Why we use localhost:3000

Why we use this localhost:3000 instead of using html file path ? I know nothing about localhost:3000 ??

Localhost will typically be however it could be different. Basically it is your machine. 3000 is the port number it will communicate on. It is pretty much an arbitrary number, so long as it isn’t clashing with something else. Your website is being hosted locally in your machine on port 3000.