Kelp buds were a type of Human seafood. They were produced by kelp, an algae, which could be found in Earth's oceans and was sometimes informally referred to as "seaweed" by Humans.
When she was growing up, Keiko O'Brien regularly ate kelp buds, plankton loaf, and sea berries for breakfast, a typical Japanese meal. However, her husband Miles did not share her affinity for seafood, preferring foods such as bacon and eggs. (TNG: "The Wounded")