When summer's heat is at its peak, the sweat can start to pour from every pore.
Normally, this wet (and stinky) process is the body's way of cooling itself off, but when high humidity turns that heat sticky, you can start to feel uncomfortable because the sweat has nowhere to go.
The higher the humidity, the more water vapor the air contains, and warm air can hold more moisture than cooler air. As humidity starts to rise, the sweat beading up on your forehead can't evaporate and provide cooling relief, because the air is already so full of water vapor, it can't take in any more.
In extreme situations, such as major heat waves, the body can become so overheated that fatigue can set in and vulnerable people, such as the elderly, may die.