What do I do if i can't download itunes on my chromebook to get rid of 'other' storage?

Every time i download a new app on my phone, i tend to see the fact that i have little to no storage left on my phone. I plan to fix this problem very soon and i need guidance. I don't know how to get rid of it. BTW: What is the other in my storage? It has frustrated my for a year and a month now and i'm not sure how to fix it. Is there another way i can get rid of the 'other' in my storage without the use of a computer/Ipad? Also, how do you back up an iphone?