Bandwidth is a control mechanism or a way to control the costs of steem blokchain. Because, everything you do (posts, voting, comments, etc.) will consume a small amount of your bandwidth. Every user has a limited amount of bandwidth to use every week. Users with more Steem (Steem Power) power will have a higher amount of bandwidth.
You can check how much bandwidth you currently have based on the current limit at https://steemd.com/@youraccountname
Speaking of how many, usually everyone's bandwidth allowance is quite high, and users can use the network freely without interruptions (constraints). Sometimes when blockchain becomes busy because of the large number of users, each individual's bandwidth allowance may go down until the network becomes less busy.