It's a userfriendly web client for the latest OpenAI chatCompletion API with additional features compared to the official chat.openai.com client (mainly: "Share chats" & better param customization), it's Open Source and can be run locally or self-hosted easily. It's built in one of the most popular frontend tech stacks of the last years. Not more and not less than that.
SlickGPT passes message history including a "context" or "system" message which it clearly explains to the user at the start of every chat. This is probably the most important thing the official chat.openai.com client fails to explain or expose to users directly.
I get that fear and it's fair. SlickGPT is OS, self-hostable and just runs locally, so you can check the code and run your own instance if you don't trust that ours running on Vercel is on-par with the one in the repo. You can also put a soft and hard cap on your API quota on the OpenAI page and monitor it to verify that only you are using it.