A base URL is the starting web address an app uses when it sends requests to an API. The app adds a shorter path to that address to reach a particular action or piece of information.
Imagine a building's street address followed by a room number. The base URL points to the building, and the added path points to the place inside it.
If the base URL is wrong, requests may go to the wrong service or fail to connect. Use the address provided by the API service rather than guessing one.