I was reading cumulative distribution function from sheldon ross and I had a doubt regarding one of it's properties.
The following are properties of the cumulative distribution function for a random variable X
(1)F is a non-decreasing function, if $a\leq b$ then $F(a) \leq F(b)$
(2)$lim_{b \rightarrow \infty}F(b)=1$
(3)$lim_{b\rightarrow -\infty}F(b)=0$
(4)F is right continous. That is for any b and any decreasing sequence $b_n$ , $n \geq 1$, that converges to b, $lim_{n\rightarrow \infty}F(b_n)=F(b)$
What does this $4^{th}$ property actually mean can anyone explain?
