The phrase "mustard seeds" refers to very small seeds that come from the mustard plant. It is often used figuratively to represent something very tiny and insignificant. Full definition
Your good book says that faith as a grain of mustard seed can move mountains. (religion.blogs.cnn.com)
Move onion mixture to side of pan, and add mustard seed mixture in center. (vegetariantimes.com)
For treating fever, just soak a full tablespoon of dried mustard seeds in a cup of warm water for about five minutes before consuming it. (herbazest.com)