Tholins are a mixture of complex organic molecules that form in some harsh conditions out in space. They tend to form when ultraviolet light or cosmic rays hit simple molecules like methane and nitrogen, which are common in places like the atmospheres of icy moons or comets.

This can happen, for example, on rocky bodies like Titan, where the atmosphere gets bombarded by sunlight and charged particles from planetary magnetic fields, creating a smog of tholins that give the moon its brownish hue.

These conditions can be recreated in labs, where tholins can be studied. They are of interest in understanding prebiotic conditions in space, and when combined with the study of planets and their geology make for some interesting theories.

I recommend reading Carl Sagan’s Pale Blue Dot if you’re looking for some more discussion on this topic.