It's shown in that screenshot in the "Overcoming your objection" section of the article, next to the fish tank graphic: "If you've already reached your quota,

It's shown in that screenshot in the "Overcoming your objection" section of the article, next to the fish tank graphic: "If you've already reached your quota, this means that: * Your Dropbox will stop syncing * You won't be able to add any new files to your Dropbox * You can't restore any files removed from your Dropbox"