random2 is a function used in Crawl's code to select a random number. It seems to be defined in the following way:
and returns a random number from 0 to max-1. Therefore, random2(2) returns either 0 or 1, which is how the coinflip function is defined.
random2 is defined in random.cc, along with many other important random functions. Most calculations of damage, quantities, and properties, will use it, so wiki editors who are looking into the source to find out these values should familiarise themself with the function.