Bandwidth points are the number of usable bytes for an account per day. In any given period of time, the entire network handles a fixed amount of bandwidth. The ratio of bandwidth points in an account to the bandwidth capacity of TRON’s network equals the ratio of frozen balance in an account to frozen balance on the entire network. Having TRON bandwidth allows you to perform transactions on the TRON mainnet without paying any gas fees.
For more information, please refer here