They look like normal French fries. However, they somewhat resemble seaweed with the way they are shaped. They have had different appearances throughout the series.
They are sometimes served in small light red paper boxes that have the Krusty Krab's initials imprinted on them in golden text, while at other times they are served in a white and red checkerboard paper tray.
Role in series
SpongeBob makes kelp fries as part of a combo meal.