Randomness in Inform (and in most computing) works by having a mathematical equation that produces a pseudorandom sequence of numbers based on a "seed" number. The sequence is only seemingly random because it's always the same if you use the same seed number, but there's no immediately visible pattern and it's practically as good as true randomness for most purposes.

For example, if you think of the pseudorandom number generator as a dice thrower, a seed of 8729 might produce results 4, 2, 6, 1, 2, 2, 5, ... and so on, always in this order. Inform's debugging option sets the seed to some fixed value which is always the same for each playthrough, so the sequence of randomness results is always the same. If the option is not used the seed itself is randomized at the start (I don't know the exact details) so that the randomness sequence itself is unpredictable.

The important part here is that Inform uses the same sequence for all randomness checks. It means that if you have two dice in the game, a red die and a blue die, their randomness isn't locked separately. If you have Inform lock randomness and throw first the red die twice and then the blue die twice, the red die's results are 4 and 2, then the blue die's results are 6 and 1. If you restart and throw the blue die first, it shows a 4 (and then a 2 and so on). So the random results are picked from the same sequence and the next die roll's result is set regardless of which die you choose to throw.

With Aaron's extension you can lock randomness in two ways: you can make it so that a die's throws are random but predictable independent of other random events in the story. In other words, you can guarantee that the red die always shows 4, 2, 6, 1, ... in this order regardless of how many times you've thrown the blue die. The other is a special case of the first one: by always resetting the seed, you can at any point lock the die so that it

always throws a 4 until you "release" the lock and it starts to act randomly again.

I hope that's more helpful than confusing.