Many times I saw the phrase something 101, such as Microsoft Excel 101. What exactly does it mean?
(chiefly US, postpositive) Basic, beginner, starting from scratch.
Geology 101 tells us that you can't build a reservoir on sandstone.
In universities courses are (usually) marked by numbers, since they are hard to remember by name. First number corresponds to study year this course should be taken in, followed by 2 (or 3) course id. Usually the same subject course has greater id if they have to be taken in the same year. Therefore lower ids are usually assigned to basic courses. So, most basic course would be 101 then as the first 1 is for the 1st year & 01 is first the 1st course.