What does phonon mediated absorption of photon to excite electron from valence band to conduction band mean?

In a so-called indirect semiconductor, the participation of a phonon in the photon absorption is essential for the conservation of momentum necessary for a transition of an electron from the top of the valence band to a minimum of the conduction band which energetically corresponds to the band gap energy. In the k-space of the energy-wave vector electron dispersion relations of an indirect semiconductor, these extrema lie at different positions which means that a large momentum has to be supplied to the electron to make a band gap transition. Photons which supply the necessary energy cannot at the same time supply the necessary momentum because photon momenta are comparatively small. This momentum can, however, be supplied by a phonon which has a small energy and a large momentum. This is how the band gap energy photon absorption is possible in indirect semiconductors like silicon and germanium. In direct semiconductors, like GaAs and other III-V compounds, such a phonon participation in light absorption is not necessary because in k-space the minimum of the conduction band lies directly above the maximum of the valence band.