Mount Sumi is a sacred mountain in Buddhism. It is considered as the pillar and the center of the whole world. Therefore, the word "sumeru" is often used to mean something very huge or important in Buddhism. And "mustard seed" is a very tiny seed, so it is used here to mean very tiny things.
This metaphor is mainly used in Buddhism to describe the state of mind and realm of a yogi. It means that through continuous practice, a yogi can gradually enlarge his state of mind, have a broader and deeper realm, and at the same time be able to see the deeper and more comprehensive nature of things, thus narrowing the original huge world into his own heart, making his state of mind as small as mustard seed, and achieving the state of internal and external integration.