How random number generators work, and when to trust them
A random number generator produces values with no predictable pattern, which is exactly what you want for a fair draw, a dice roll, sampling, shuffling or picking a winner. The tool above generates numbers in whatever range you set, so you can leave a choice to genuine chance instead of a biased human guess, which is more defensible whenever fairness matters.
Pseudo-random versus true random. Most everyday generators, including those built into software, are "pseudo-random": they run a formula from a starting seed that produces a stream statistically indistinguishable from random for normal use. True random generators instead draw from physical noise. For games, raffles, sampling and decisions, pseudo-random is perfectly fine; the difference only matters at the extremes.
The one place not to use a basic generator. Security is the exception. Passwords, encryption keys and tokens must come from a cryptographically secure generator, because ordinary ones can be predicted if someone learns the seed. For any everyday "pick a number, pick a winner, shuffle this list" job, a standard generator like the one above is exactly the right tool; just do not use a casual one to mint secrets.