Astudent made a model of isostasy by placing a block of wood in a beaker of water. what does the water in the beaker represent in the model?

Isostasy is the concept used to explain the dynamics of the earth's upper layers. It is considered that there is a gravitational equilibrium that exists between the earth's upper most layer, the crust, and the next layer, the mantle. The crust is said to "float" on the mantle.
Therefore, in this model, the block of wood represents the crust and the water represents the earth's mantle.