If a connection is congested or saturated it means it is performing at maximum capacity, any further traffic will cause latency. You may experience drastic drops in your network, unacceptable packet delays and session disruptions causing jittered screens.
Congestion can be caused by too many clients attaching to a single Access Point, large downloads being undertaken by several people at the expense of others, external interference from other noise sources or other access points on the same channel. Latency can also be experienced due to hardware, server and access point locations.
Potential solution: It may simply be your network requires optimisation. In more complex situations, It could mean your network needs redesigning, underpowered equipment replaced or it could be time to upgrade the link speeds.
A Site Survey to audit your wireless network is advised (including a spectrum analysis) to identify latency issues.