This phrase is from the book of Daniel. His audience would have been familiar with this passage and the prophecy about the abomination entering the temple and defiling it. Alternate translation: "the shameful thing that defiles the things of God" (See: [[rc://en/ta/man/translate/figs-metaphor]])
Jesus' audience would have known that this refers to the temple. This can be made explicit. Alternate translation: "standing in the temple, where it should not be standing" (See: [[rc://en/ta/man/translate/figs-explicit]])
Mark added this to get the readers' attention, so that they would think about what Jesus meant when he spoke about the abomination of desolation standing where it should not be standing. Alternate translation: "may everyone who reads this understand what it means" (See: [[rc://en/ta/man/translate/figs-explicit]])