It isn't random it's a random-like number generating algorithm and it's based on a number to generate. If you set that random number to something like the system time the numbers are more close to random, but if you use these numbers to lets say, an encryption algorithm, is the attacker knows WHEN you generate the random numbers and the algorithm you use, then it is more possible that your encryption will break.
The only way to generate true random numbers is to measure something natural, for example voltage levels or have a microphone picking up sounds somewhere or something like that.
