System Center Operations Manager Server Managers monitor other Servers, Clients, Applications, and Devices with Agents by exchange a predictable set of data, which is sent periodically by the Agent to the Server Manager. This set of data is called Heartbeat.
 

By default the heartbeat period is 60 seconds, and a Management Server can miss 3 heartbeats. When four heartbeats are missed in a row, an alert will be sent to indicate that the Agent is no longer available and a diagnose ping is sent to the Agent. If the ping wouldn't reach its destination then a second alert is generated to indicate the Agent is no longer reachable.

the picture below illustrates the scenario, in which the agent is identified no longer available. It is important to notice that the heartbeat interval (period) and the number of missed heartbeats tolerated by the Management Server can both be modified (In the Operations console -> Administration button, In the Administration pane -> expand Administration and click Settings, In the Settings pane-> expand Type: Agent, right-click Heartbeat, and click Properties)