The phrase "water seeks its own level" is a metaphor for how things equalize.
In physics it stands for hydrostatics and is influenced by atmospheric pressure, water pressure depending on depth and water's density. A good example for this process are 'Communicating Vesels', where several containers connected through pipes are filled with liquid. That liquid will reach the same level in all parts of the system, regardless of what the lowest point is of the pipes.
The artist Allan Wexler created a brilliant artwork Coffee Seeks its Own Level about this phenomenon:
"Coffee Seeks its Own Level choreographs group dynamics. If one person alone lifts his cup, coffee overflows the other three cups. All four people need to coordinate their actions and lift simultaneously."