/dev/urandom

A 'device' whose sole purpose is to quickly provide random data.

To see it your yourself:

$ cat /dev/urandom

(remember to press Ctrl-C when done, which sends the INT signal to cat, which then exits)

There is also /dev/random, which provides better (more random) data, but will be a lot slower.

The data may be from an actual, random number generating device, but on most home computers will be pseudorandom.