Here's a breakdown:

* Weight of the Earth's crust: This includes the weight of all the rocks, soil, water, and ice on the surface.

* Buoyancy force: This is the upward force exerted by the denser mantle on the less dense crust. It's similar to how a boat floats on water.

Imagine the Earth's crust like a giant iceberg floating on the denser mantle. The crust floats at a level where its weight is equal to the weight of the mantle it displaces. This is isostatic balance.

Key points to remember:

* Isostasy is a state of equilibrium, not a fixed state. Changes in the crust's weight (like the deposition of sediments or the erosion of mountains) will cause the crust to adjust vertically until balance is restored.

* Isostasy is a fundamental concept in geology and helps explain the formation of mountains, the distribution of landmasses, and the movement of tectonic plates.