A server tick is a small, recurring increment of time. These ticks cannot be manipulated by the player to come faster or slower.
All information, actions, and visuals are based on server ticks. If a server gets overloaded with information (such as at a Hamidon Raid or official event) it will slow down server ticks in order to pass all the required information back and forth between the servers and the clients involved. This will cause visual and power lag, where powers take longer to recharge than normal and characters rubberband as the server comes back in sync with the client, which is processing information faster than the server.