The Earth’s lithosphere, which is the crust and upper most part of the upper mantle, is broken into tectonic plates which float in the more viscous asthenosphere, which is the remaining part of the upper mantle (under the lithosphere.)
Iso (same) stasis (still). Isostasy is the process that restores isostatic equilibrium; to restore balance/buoyancy. A larger weight (i.e. a mountain) will cause the plate to sink down. A depletion of the crust (erosion) will cause the land (plate) to rise. As a mountaintop erodes, it will rise (less weight), only to be eroded further, thus maintain the balance (buoyancy) of the lithosphere upon the asthenosphere. Think of an iceberg which will sink the heavier it is and will rise the lighter it is. A rising mountain could be evidence of a decrease in weight due to erosion or if the mountains are rising faster than they are eroding, this is isostasy at work; the rising topography buoys toward equilibrium amidst the surrounding areas/plates. If the mountain range is still Not in isostatic equilibrium, the cause could be other factors:
Other contributing factors to the overall isostasy gravitational and polar shift and the merging and converging of plates.
A more simple example. A mountain range could be the effort to fill a shallow section of the asthenosphere. Isostasy is the Earth’s process of maintaining the balance of outer shell. The idea is that the combination of lithosphere and asthenosphere will weigh the same everywhere on Earth: this would be total isostatic equilibrium. When the liquid asthenosphere undergoes density/volume shifts, lithosphere mass will make up the difference (mountains or dense terra). When the lithosphere gains or loses weight, it will sink and displace the underlying asthenosphere, which will then potentially cause imbalance in surrounding areas, which must also be corrected by isostasy.