Monument is a term for some object that is placed to provide a durable record of a survey. Once monuments are put in place, they have legal standing if disputes ever arise. A monument may be a length of steel rebar that has been hammered into the ground, or a post that was set, or a pile of rocks (a "cairn") that marked a position, or any number of other things that may have happened to be at hand.
A "concrete monument" usually refers to a concrete post or a cylinder of concrete about a foot in diameter that was poured into a hole in the ground with its top flush with or below the surface. If a disk is "set into the top of a concrete monument" it means that such a concrete cylinder was poured, and then the survey disk was set into its top. The whole thing may then be covered over with soil, lying below the surface out of harm's way but available if it ever needs to be used.