In the context of MCNP, "2M" and "20R" refer to specific notations where "M" stands for multiply, "R" for repeat, and "I" for interpolate. For example, "20M" indicates multiplying a number by 20, while "20R" means repeating a number 20 times. This shorthand simplifies the representation of long lists of numbers, making it easier to write tally bins. The discussion emphasizes the utility of these notations in reducing effort when dealing with numerical data. Understanding these terms is crucial for efficient communication in MCNP-related tasks.