What does "Random" mean in a Computer?
In a computer, "random" can mean two completely different things:
1: random may mean "unpredictable," in the sense that a value could be any value, so we don't know what we'll get. In this case, we're normally talking, ironically, about very predictable, deterministic pseudo-random number generators (PRNG). Whenever we talk about "RNG" in games, we're talking about this type of random. Pseudo-random number generators use a number called a "seed" to generate another number from it, such that the next random value after the "seed" is ALWAYS the same number. In other words, it's not true randomness, since you always know what the next value is going to be if you know what the current seed is. True randomness is possible in computers, but isn't achieved through software; instead special hardware is used to derive a random number from the environment. A simple example would be measuring the precise temperature of the CPU, which is sufficiently random and doesn't depend on the state of any software.
2: random may mean "arbitrary," in the sense that a value could be any value, so we have to be prepared for anything. For example, when we talk about Random Access Memory (RAM), the "random" in "RAM" means that any address in memory may be accessed at any time, arbitrarily. The opposite would be, for example, sequential access memory, where an address can only be accessed if you have accessed the address that comes before it first. In RAM, you can access the 10th address any time you want, while in SAM you need to access the addresses 1 through 9 to access the 10th. At a software level, we see random access in arrays and sequential access in linked lists.
As we can see, "random" fundamentally means "it could be any value," but there are two different ways to use this notion. The first being that we don't know what we're going to get, so we can't depend on it. It's a lottery. It's a roguelike. The second being that we don't know what will happen, so we need to be able to handle everything. In particular, when we talk about untrusted input, or user input, the idea that a system must be able to take random input and it won't crash nor its security will be compromised just by that.