For the benefit of others not au fait with social media culture, Wikipedia describes thirst traps as ‘a type of social media post intended to entice viewers sexually. It refers to a viewer’s “thirst”, a colloquialism likening sexual frustration to dehydration, implying desperation, with the afflicted individual being described as “thirsty.”’
You're thirsty if you're horny, typically for a particular person but not always.
You're thirsting over someone if you're horny for someone in particular.
A thirst trap is an act (or video, or Instagram post, etc.) that intends to make others thirst over you. It's like a mating display, but with ulterior motives, if you want to look at it that way.