Since you're a beginner I assume you are working with gdscript.
Every script can be a class by itself, and can even have inner classes inside these scripts. Whenever you use the keyword
extends in a script (which is a necessity anyway), you are extending from either a built-in class, node or another script. To extend from another script, you would do something like this.
extends "script file path"
To easily get the (absolute)path of the script you want to extend from you can find it in the file explorer on the left, right click it and press
Copy Path. Then just paste it in quotations after the
extends keyword. Note that you can only extend from one thing in one script at a time.
So In your example, if the path to the mob script/class is
res://mob.gd, then in your fish script you'd type this at the top of the script.
You can learn more about the fabulous gdscript language here in the Godot docs.