That's random in maths, we're not talking maths here. In the real world, random is a value that's hard to predict. For example a dice throw has a high quality of random in normal circumstances.

A low quality random source would depend on some variables that make it easier to predict the next random number (perhaps just statistically). The worst quality random source would be pseudo-random numbers, where the value of each next random number directly follows from the previous value and perhaps some secret second value.

It might be distributed like a true random number, but eventually the secret could be guessed and the numbers would become predictable.