Just got home from doctor. She said I have heat exhaustion. Looked it up online and now more confused that ever. Can you explain what it means to have it?

Confused also :) Heat exhaustion is not a disease that a person has, but something that happens to you when in a hot enviroment when you can't keep cool and your body temperature elevates to an unsafe level. If the body temp keeps going up and your body decompensates, a more severe state of heat stroke can occur. True cases of heat exhaustion require rapid medical attention.