The full text is "Blooming flowers inside the wall and fragrance outside the wall". It should be derived from poems such as "A branch of red apricot comes out of the wall".
That is to say, the flowers planted in the wall are mediocre and unpopular inside the high wall, but their fragrance spreads outside the wall, giving people a beautiful feeling and infinite yearning.
It is now used to describe things that are unpopular in their place of origin, such as domestically, but are sought after elsewhere.
For example, a movie may have average domestic box office results, but it is popular abroad, with high box office and good reputation. It can be called "flowers blooming inside the wall and fragrance outside the wall".