It's a mathematical term, but it's often used as an expression meaning that something is aimed at the stupidest, least educated or least sophisticated members of its audience. It's very rare for it to be positive. The expression is most commonly used in entertainment, advertising or politics:
"This movie is pure lowest common denominator stuff" - the film is full of dumb populist elemen
Let's say that you are a teacher. You teach algebra.
There are 30 students:
10 are very smart. 10 are average ( so so) 10 are stupid (like me)
What do you do? If you teach for the smart ones, the average and the stupid will not understand; if you teach for the average. the very smart will be bored, and the stupid will not understand.