Everything you do (posts, voting, comments, etc.) will consume a small amount of your bandwidth. Every user has a limited amount of bandwidth to be used every week. Users with more Steem (Steem Power) power will have a higher amount of bandwidth.

You can check how much bandwidth you have currently based on the current limit at https://steemd.com/@ youraccountname.

Usually everyone's bandwidth allowance is quite high, and users can use the network freely without interruptions (constraints). Sometimes when blockchain gets busy because of the large number of users, each person's individual bandwidth allowance may go down until the network becomes less busy.