蒸れる is "to be humid." So, if you have a humid hot, you have weather that is hot and humid (or you can just say humid, because that implies hotness too, usually). This is a useful word for summers in Japan, since you'll complain about this all the time.
The reading comes from 蒸れる and 暑い. Know those and you know this too.
This summer is definitely more humid than it was last year.