What does moho stand for?
It's the Mohorovic Discontinuity, the boundary between the crust and the mantle.
Scientists are not sure exactly what the Moho really is, but seismic evidence suggests that there is a change in the composition of the bedrock. At the high pressures deep inside the Earth, different minerals and therefore different rocks become stable, so such a change in composition is quite plausible.
For more about the Moho, see here: