Bandwidth on Steemit is your ability as a user to perform an action that is recorded in the blockchain. This means posting, commenting, editing, voting, and even collecting your money all consume the bandwidth you have as a user.
I'm sure you could see how not having enough bandwidth could be a problem then... without any bandwidth you're basically unable to do anything on the Steemit platform and that makes this site breakdown and makes your chances at making any money or growing your influence nearly impossible.
Thankfully for all of us, however, bandwidth slowly regenerates on your account. Along with that, your bandwidth will increase if you have more SP, which gives Steemit users another reason to power up. You can look at both the amount of bandwidth you have and the amount you have used at steemit.com/@username.
Hopefully this helps you understand another part of this vast Steemit platform. If you found this helpful or interesting let me know if the comments and follow along for more Steemit tips and info!