It's a forum for people at tech companies to anonymously talk to other people at their companies in a private forum, or with people at other companies publicly to all members. They validate employment using your email address. As could be expected many of the topics devolve into toxic messes, but there is some useful discussion.
Currently to register in the app you have to select US or South Korea. They are setting a strong expectation that you should only do so from one of those countries, which makes it seem unlikely to me that I will find any colleagues -most of move would not think to fake their location. They also won't let you leave a review without a location in the US or South Korea. Makes it all a bit pointless if your company is not in those countries.
One of the most toxic environments on the internet, filled with people who define themselves by their employment at big tech, and how large their salaries are. Think WallStreetBets meets tech but with less self-deprecation and emotional maturity, if you can imagine it.