It is a website (https://barter.vg) that links to your Steam profile. You can then publish a list of games for which you have unredeemed keys (from bundles or something similar), and you can add a wish list of games you would want (or sync with your Steam wish list).
You can then start looking for matches (people who want stuff you have and who have stuff you want) and send / receive offers. Once you both agree on an offer, you contact each other trough Steam and exchange keys, redeeming immediately to make sure the key works. Afterwards you indicate that the trade went well on each others public profile. Scammers are kept at bay by a system of trust: people who have done a lot of trades already without disputes can clearly be trusted. This is why it is linked to your Steam profile, to make it harder to make burner accounts.
When you first start trading, it is an unwritten rule that you reveal your key first to the other party (assuming they already have done a lot of trusted trades). My first experiences were all super nice, people are very friendly and supportive, it's like this community of game collectors who like to find good deals and help each other play the games they want to play. I highly recommend it!
I am at 9 successful trades right now, since joining a few months ago! (my profile to give you an idea: https://barter.vg/u/a46a/)
I'm glad that it seems sufficiently different from Ghost of Tsushima. I kind of dislike how most sequels these days are almost the same game with some small improvements in graphics and gameplay (Horizon Forbidden West, Spider-man 2). With the new character and time period, this seems like a more substantial change!