[B >: A] is a lower type bound. It means that B is constrained to be a supertype of A.
Similarly [B <: A] is an upper type bound, meaning that B is constrained to be a subtype of A.
In the example you’ve shown, you’re allowed to push an element of type B onto a stack containing A elements, but the result is a stack of B elements.
The page where you saw this actually has a link to another page about lower type bounds, which includes an example showing the effect.