You can get good random numbers from many things. Easiest might be a temperature sensor. Imagine a temperature sensor with a digital display, with 20 digits of precision. So it might say the temperature is 72.54472792884724655763 F  (about 22C). But the next time you look it might say 72.47798392534y7328... or, if it was a very expensive thermometer it might say 72.545076543277...

The very last digits will change very fast, and very randomly. They are good sources of randomness.

But, most random numbers used in computers come from pseudo-random number generators. These are mathematical programs which are designed to produce numbers that are as random as possible. Some of these are not good for cryptography, and you can crack them, and anything encrypted with them is not secure.