What is seismic stretch in geophysical method ?
I believe you are referring to - as @ye-ti-800 has already pointed out - the apparent stretch that results from the NMO correction applied to CMP gathers. There is a great tutorial supplied by a Leading Edge article that may be accessed here: https://wiki.seg.org/wiki/Step-by-step_NMO_correction. It is important to note, though, that the article does not go into details about stretch mute. The article just goes into the specific details of the "mechanics" occurring during a conventional NMO correction and accompanied stretch that results from it.