上坟 means 'to visit a grave to pay respects to the dead,' a common ritual during Qingming Festival or on death anniversaries. It is a separable verb (离合词): the object 坟 can be separated from 上 with measure words, durations, or other elements inserted (e.g., 上了一次坟 'visited the grave once'). You do not add an object after the whole word; the grave is already part of the verb-object compound.