What do the bandwidth limitations mean?
Changes made to your files in the Dropbox desktop app are reflected everywhere you access Dropbox. Your upload rate affects how fast this happens.
When you add or change files on dropbox.com or anywhere else you access your files in Dropbox, those changes are reflected in the Dropbox desktop app. Your download rate affects how fast this happens.
Both of these actions use your internet bandwidth. Your internet has a limited amount of bandwidth, and several desktop apps may use your internet bandwidth at the same time.