Congestion control is a mechanism for handling potential communication errors due to congestion on a computer network. When congestion exists the speed of the network connection is adversely affected. Congestion occurs when too many people are trying to send too much data at the same time on the same computer network. This congestion can be measured using a network latency metric.
In the Transmission Control Protocol (TCP), a congestion window is used to avoid and control congestion. The congestion window size is controlled by an additive increase and multiplicative decrease approach. For example, when an acknowledgement timeout occurs the congestion window size is reset, the receiver limit is set to half, and a slow start is initiated.
Please see Wikipedia for more details on TCP congestion-avoidance algorithm.