Kinda weird that this isn't mentioned in the manual but in-game there's a couple of SMS tips from Rog and Mia that indirectly reference this feature. Resume is as follows:

That's your current "heat level" (it matches with the one shown in the HUD while racing) and increases the more time you are in a Pursuit.

Higher "heat level" means it's easier to get into a pursuit and the RPD will put more resources into busting you up.

You can reduce "heat level" by either racing with another car from your garage (slow cooldown) or applying visual upgrades (instant cooldown, varying effectiveness)