Heartbeats are used to help keep the user's session alive. Sessions have a timeout (that is by default 20 minutes). If you game isn't generating enough API traffic, you might find that the session would terminate due to inactivity. The heartbeat prevents that from happening - so that if your app/game is still running, even if it isn't sending requests to braincloud, the client library will send a request every <x> seconds to keep the session alive for you.

Note - the # of seconds for a heartbeat is calculated at 85% of the session expiry. So if your session expiry is sent to 1200 seconds (which is the default = 20 minutes), the heartbeat would be set to 1,020 seconds, or about 17 minutes. 

Note - these heartbeats only get sent if your game is running, but it isn't making brainCloud API calls. In practice, these are a very small percentage of your overall API usage... (I just checked one of the games in the system that's done 4M API calls so far today, and it had only done 1,086 heartbeats - or 0.02715 % of calls!