Ports and what they mean as a process:
Sometimes working on react app, you run into issues like:
Something is already running on port 3000.
To handle this:
For Linux/Mac OS search (sudo) run this in the terminal:
$ lsof -i tcp:3000
$ kill -9 PID
On Windows:
netstat -ano | findstr :3000
tskill typeyourPIDhere
change tskill for taskkill in git bash
Source: referring to this SO: https://stackoverflow.com/a/45130296