The most common random number generators are based on so-called Linear congruential relations of the type \begin{equation*} N_i=(aN_{i-1}+c) \mathrm{MOD} (M), \end{equation*} which yield a number in the interval [0,1] through \begin{equation*} x_i=N_i/M \end{equation*}
The number M is called the period and it should be as large as possible and N_0 is the starting value, or seed. The function \mathrm{MOD} means the remainder, that is if we were to evaluate (13)\mathrm{MOD}(9) , the outcome is the remainder of the division 13/9 , namely 4 .