This is the holiest place for Christians in Jerusalem as it is where they believe Isa (عليه السلام) [Jesus] was crucified. This concept is rejected by Muslims who believe Isa (عليه السلام) did not die but was taken up to the heavens by Allah (ﷻ) and will return towards the end of time.
Featured image:https://www.viator.com/Jerusalem-attractions/Church-of-the-Holy-Sepulchre/d921-a10092