Just Launched refers to sellers which have joined Amazon marketplace recently. Amazon highlights those with a "Just Launched" text line under their seller name.
However it is often for just launched sellers to be scam sellers.
If a just launched seller has thousands of products listed (which you can check by clicking on their profile), and prices offered look too good to be true, then do not buy from them.
You can see this in the attached screenshot example - brand new seller, no previous feedback reviews, and a price far cheaper than anyone else.
Buying from sellers with positive feedback reviews, at least a few dozen of them, is strongly advised.
We've written in How to spot a scam seller that sellers with prices too good to be true should not be trusted. Especially if it is a brand new seller with no feedback.