I'm currently freshing up on math for my computational biology seminar. Now I have a question concerning the following density function that is used to calculate the signaling time in a signaling network of a cell. My book states that this time is based upon the density function;
$$f(t)=\frac{X(t)}{\int_{0}^{\infty}X(s)ds},$$
where $X(t)$ is the count of bound molecules.
How am I to interpret this density function? Do I guess right that $f(t)$ yields the probability of $X(t)$?