Bestseller basically means that you sell better than any other books, usually as part of a bestseller in a category, which is fine. (If you are a kindle or paperback bestseller in the overall sales, you need to be doing a training session with Jay to let us all know how you did it!)
Some people consider themselves a bestseller if they get in the bestseller list (top 100), others only if they are first page (top 20) or better. Obviously, you need to hit #1 to be a #1 bestseller.
If your book hasn’t sold yet, then I personally wouldn’t call it a best seller even if it is ranked in the #1 spot, but if it’s already there then I’d recommend doing a little promotion to get a few sales and that way you definitely will be the best seller, and you also have a better chance of remaining that way rather than being randomly bumped around.
Putting bestseller seals on the cover of your book can help with social proof, but they aren’t anything official; Jay has offered them as part of Book Cover Genius and as bonuses for a few of their other products so if you’ve bought anything it might be worth taking a look through what was offered. You could also just make your own.