What's the difference between a ledger balance and an available balance in a back account?

I went to Staples yesterday to try and buy some supplies for my college cles that start Tuesday. Went I tried to swipe my debit card I got declined. When I went to my bank it said I had over $100 in a ledger account and an available balance of $20.00. Why couldn't I buy my supplies and how do I get my ledger balance to become my available balance?