This is the 3439th most frequent Japanese word.
熱帯
Tropical region.
Here, "熱帯" (nettai) is used as an adjective to describe regions characterized by tropical climate and location.
熱帯地域では生物多様性が非常に豊かです。
In tropical regions, biodiversity is extremely rich.
In this sentence, "熱帯" is used as a compound word "熱帯魚" meaning "tropical fish", referring to fish that inhabit warm waters.
熱帯魚を飼うのは初心者には少し難しいかもしれません。
Keeping tropical fish might be somewhat challenging for beginners.
Here, "熱帯" is used as a noun by itself to denote the tropics, geographical regions with warm climates.