In certain situation you can gain heat. The more heat you have the easier it is for cops and rival Informants to find you. When heat is above 0 cops an rival Informants have a higher chance of finding you.
Heat does passively decrease over time but not lower then 10. If evidence is collected against your situation your heat can’t decrease lower then 20.
If you have lower heat then your decrease limit it will not decrease further until you find a way to lower the limit.