Lag or latency is a very common term in multiplayer games, as it refers to the late response of a server due to hard/high data transfer over the network. Lag is especially noticable on tight situations where a lot of data needs to be computed and transferred to the clients. When lag occurs, all actions a client performs will be delayed by a small or long period or even get repeated many times before really being executed. A popular example is when you are walking, and lag makes you walk the same distance over and over again without you being able to stop it even when are doing nothing or standing idle.
In many cases, too much lag will cause players to disconnect due to the game being unplayable at a normal level.

When it occurs

  • Lag can occur simply by the server, whereas there's a lot of network traffic to calculate, and thus sending late responses to all clients. There's basically nothing you can do about it unless waiting for the server to have calculated everything. Tips are for the responsible persons of the server to disable flashlights (mp_flashlight 0) and disable voice communication.
  • Lag can occur when you are downloading or even worse, uploading. Never play an online multiplayer game during an upload. Doing so will give you an average latency higher than 500, thus causing the game to be completely unplayable.
  • Search for servers that are close to your country. The shortest way to transfer data will always give the best result! Connecting to a server in america while living in Europe is a very bad idea. Likewise, connecting from an European server to an Asian server is also a bad idea. Generally, you should always try to search for a server in the same continent as yours.