It means that the flowers planted in the wall are inconspicuous and unpopular in the high wall, but the fragrance spreads outside the wall, giving people a good feeling and infinite yearning.

Metaphor is talent or technological invention. Not valued here, but widely known elsewhere. Metaphor means that people have made achievements, but they don't know internally, and they know externally first. Metaphorical achievements are not valued by this department, but appreciated by other departments. By extension, some things are not as popular as in other places.

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." Flowers in the wall are fragrant outside the wall, which is a diffusion phenomenon. Molecules are constantly making random movements.