If something is runny, it is like a liquid and not thick.
Runny can make people think of things like noses when someone is sick, or of food that is too wet.
Examples:
Her nose is runny because she has a cold.
The ice cream melted and became runny.
The runny paint dripped down the wall.
The runny pudding spilled all over the table.
The river was so runny that it flooded the town.
The runny honey dripped off the toast.