How do I interpret the heart rate monitor?

While hunting monsters, I've noticed that a pseudo-heart rate monitor is displayed when the monster is nearby.

While fighting them, I've tried to keep an eye on it to try and understand what it indicates, but I haven't been able to figure it out.

Sometimes, the heart rate spikes and flashes orange, while other times it slows down and stays blue. I'd originally assumed it flashed orange and spiked when the monster was dying as it was fighting for its life, but this hasn't always been true; sometimes it's quite slow even when the monster is limping.

What information does the heart rate monitor tell me? How should I interpret the display?

There are three statuses that the heart monitor can display, as mentioned in the question: normal, quick and orange, and slow and blue. Normal is self-explanatory, it means the monster is not in any special state.

The orange, quickened heartbeat signifies that the monster is in rage mode. This is triggered by receiving damage and, while rage mode is more frequent when the monster's health drops, it doesn't have a 100% uptime. While in rage mode, the monster becomes more aggressive, faster, and deals more damage.

The blue, slower heartbeat signifies that the monster is exhausted. This is triggered by the monster tiring itself out from constantly attacking, and can be quickened by using impact weapons, which apply exhaust damage. Once a certain amount of exhaust damage has been applied, the monster's stamina will drop by a chunk and, if low enough, will leave the monster exhausted. An exhausted monster is slower than normal, may stand still while doing nothing, and will often try to hunt prey or eat carcasses to recover stamina.

Besides the color you could also use the peaks to determine a monsters remaining health. Like the closer to flat lining means it's almost dead.

Take note though that there will be spikes in heart rates or false positives like for example: roars will cause a big spike in heart rate while certain CC like sleep will lower it.

I'm still trying to learn what certain peaks mean since they correspond to certain % health left.

I usually use the heart rate for stuff like, knowing when the monster will transition areas (very useful for Behemoth) or like when I can capture it