University of Alberta Guide/STAT/222/Formulas and Functions:Cumulative Distribution Functions

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The $F_{X}(x)\rightarrow (cdf)\,$, represents the probability that the value of a random variable $X\,$, regardless of its distribution, will be less than or equal to the value $x\,$ that is provided. For continuous random variables, $F_{X}(0)=0\,$ because $P(X=0)=0\,$ for all continuous random variables. Therefore, all cumulative distribution functions will start at zero.
Generally, $F_{X}(x)\,$ represents $P(X\leq x)\,$, however, if what is desired is $P(X>x)\,$ then $1-F_{X}(x)\,$ would be used.

CDF for an Exponential RV

Notice that as $x\,$ increases, the probability nears closer and closer to one. This is because as $x\,$ increases, the area under the curve of the PDF becomes closer and closer to one.