When a carrier reserves the right to de-prioritize the traffic of high bandwidth users. They generally have a threshold before this kicks in, although some plans are always subject to it.
After that threshold is reached, if you're on a cellular tower that is experiencing congestion, your usage might be slowed down in relation to other customers. Once the tower is no longer congested, full speeds should resume.
When on underused towers, you may never experience it. But if you're frequently in congested areas, it can be a problem - especially during peak hours.