"Mashallah" is an Arabic phrase often used by Muslims and Arabic speakers to express admiration, appreciation, or praise for something good or beautiful that has happened or is being observed. It can be translated to "God has willed it" or "As God has willed."
The response to "Mashallah" could be "Jazakallahu Khair" (جزاك الله خير), which means "May Allah reward you with goodness" or simply "Thank you." This response acknowledges the praise and expresses gratitude. However, there is no strict rule for responding to "Mashallah," and people might respond in various ways depending on the context and their cultural background.