a chaser is a cis person who fetishises and objectifies trans people, frequently in a way that's pretty transphobic once you look under the surface. A lot of the time though they're able to avoid being labelled as transphobes and postion themselves as allies because they agree that trans people have a right to exist, and don't hate trans people the same way - they're basically the mirror image of people who used to say shit like "i support gay rights because it means i get to see women kissing in public and have threesomes with bisexual girls". A pretty common element is also them taking advantage of the fact that a lot of trans people are societally vulnerable, frequently face social rejection and so are more likely to accept any form of "positive" attention, even if it comes from someone who just wants to fuck them, need money for gender affirming treatment, etcetera - so this ranges from chasers getting with trans people because trans people are more likely have lower standards on the mild end of the spectrum to on the other end, threatening to withhold gender affirming treatment from them, taking advantage of their economic situation to coerce them etcetera.

sometimes though the term is used incorrectly to just refer to people who are just attracted to trans people, without it being fetishistic or controlling

tl;dr: they're basically just guys who would be sexpats but they want to stay in the first world, so looked for a different vulnerable group at home and settled on trans people