101 usually refers to course level (how difficult or advanced the course is). Algebra 101 would be the most basic algebra course you could take, usually the first one that you would take.
Freshman and sophomore level classes are usually 100 and 200 level, junior is 300, senior is 400, and 500 and 600 are graduate level classes. So you could take Algebra 101, 201, 301, etc.
101 is also used to describe any basic information: Cooking 101. Dating 101. It means the very basic understanding you need to have in a topic area to get any better