What does (1-) mean in the context of probability mass functions?

Question

I am checking over some work I'm doing, and I noticed the following formula:

$P(X=i) $

$= P(X\le i) - P(X < i)$

$ = F(1) - F(1^-)$

I understand the concept of the second line, but I don't understand the 3rd line.

Answer 1

$F(1^-)$ is shorthand for $$ \lim_{x\to1^-}F(x) $$ which is to say, the limit of $F(x)$ as $x$ approaches $1$ from below.