An imaginary form of measurement used by telephone companies trying to determine how much data is being passed across the network on DSL. There is absolutely no conversion from "cells" to, say, kilobytes, and the amount of cell flow may vary depending on what kind of DSLAM your service is coming through. Tech support will generally use this as a be-all end-all for how good your connection is. For example, if there's a lot of cell flow they may think you're either downloading a lot of pr0n, you have a virus, or there's nothing wrong at all. If there's only a trickle of cell flow, they may think you have a firewall, you're not at home browsing actively, there's a problem, or there's nothing wrong at all.
Customer: I can't bring up any web pages.
Tech Support: Really? I see a ton of cell flow. Maybe you've got a virus.