Answer to Question 1
Hybridization of orbitals refers to the combination of atomic orbitals which produces a new (hybrid)
set of atomic orbitals.
Answer to Question 2
The four bonds in CH4 are equivalent (giving a perfect tetrahedron), which requires our equivalent
MO's. Combining a 2s and a 1s would give a different MO from one obtained by combining a 2p and a 1s, so
that a tetrahedral geometry would not be possible.