When it's very hot and uncomfortable, like in the summer or in a small, stuffy room.
Sweltering is not good because it makes people feel very uncomfortable and sweaty.
Examples:
She swelters in the hot sun.
The room swelters without any air conditioning.
He swelters in the crowded bus.
The cat swelters in the heat, just like me.
I feel like I'm sweltering in this heavy coat.
The dog swelters in the car while we shop.