A landmark number is a number that makes math simpler by virtue of it being easy to relate to and calculate with, such as the number "10." Most landmark numbers are multiples of 10.
The number system most often used for math today is base 10. According to Purple Math, base 10 means that the number system has actual numbers for one to nine, which start over at 10 by adding a one to the 10 column. (A base seven system only has numbers one through six.) Other bases are common for certain applications; for instance, computers use a binary system (base two).
Because the most common system is base 10, numbers in multiples of 10 serve as landmark numbers — as a sort of safe landing spot when doing calculations. When solving math problems, certain numbers make those calculations easier or harder. For instance, multiplying by "100" is much easier than multiplying by "4.23" or even "7."
Landmark numbers also help students understand relationships between numbers. For instance, according to Investigations, when adding two numbers that are each less than 100, students should should realize that they won't end up with a number that's more than 200, helping students to judge when they've made an error in addition.
Students can also rearrange numbers to create landmark numbers. For instance, when subtracting 13 from 21, it's more likely that a student will know how much it takes to get to 20 (7), plus the extra 1 over 20, which makes 8.