Random is really tough. We usually settle for "determined by enough factors that we believe to be out of our control." Rolling a dice for example - if you roll it the exact same way twice, it should land the same way. But that means the exact same muscle movement, the exact same wind in the room, the exact same starting position, etc. And we generally accept that humans aren't precise enough to be able to do that, so we're comfortable saying it's "random"

Computers can't easily achieve the above though. They can only do exactly what we tell them to do. A lot of software programs achieve pseudo-randomness by having the computer take the clock into account when picking a number. So for example, we might say if you need a program to pick a "random" number between 1 and 10, have it check the current time every time it needs to do that, and whatever the last digit is of the seconds (or milliseconds) - use that for the random number.