What does it mean to upgrade your bank account in neopets

It just gives you a higher level of interest. A basic account only gives you 4.5% interest. But if you have 1,000NP in the account, you can upgrade it to a Neopian Student account, giving 5.5%, all the way up to 12.5% interest on an Ultimate Riches! account though it requires a minimum of 10,000,000NP to upgrade it to that level.