Hey there! I'm happy to help!
Sedentary means you stay in one place and aren't very active. Nomadic means that you travel around a lot.
If there are a lot of available resources, the tribe will want to stay because having resources nearby will help them survive much easier. This would make them sedentary.
If there are scarce resources nearby, they will go away and look for more abundant resources, making them more nomadic.
Have a wonderful day! :D