The full text is the flowers inside the wall, and the fragrance outside the wall. It should be from the poem "An almond goes out of the wall".

In other words, the flowers planted in the wall, inside the high wall, are unremarkable and unpopular, but the fragrance has spread outside the wall, giving people a good feeling and infinite yearning.

Now it is used to describe something that is unpopular in the country of origin but popular elsewhere.

For example, a movie has an average performance at the domestic box office, but it is very popular abroad, with a high box office and good reputation. It can be described as "flowers inside the wall, fragrance outside the wall."