Thirst trap is a slang term used to refer to posts or images that are sexy or flirty in nature and designed to sexually entice others.
Thirst is, of course, a physical drive or craving. The term "thirst trap" implies that the images induce a craving or meet a need, in this case, a sexual one.
The term arose in the 2010s in response to social media's "selfie culture."