This is a phrase you can use to describe Tokyo (or New Orleans) in the summer. It’s a nice, little non-idiomatic four-character compound – high, hot, many, moisture: high temperatures, lots of moisture.
The pronunciation is こうおんたしつ.
The usage is simple. You can attach this onto other words with の, so 高温多湿の場所 or 高温多湿の国. Can’t think of much else you would describe with this phrase.
(Note: This was written before this tease-of-an-autumn "cold" spell. It will be above 30 again later this week.)