Discretely means in a clearly separate way. If two jobs are handled discretely, they stay in their own piles; nobody stirs them into one mess. If lights come on discretely, they jump on one after another, not as a slow fade. Maths and science use it the same way: values that change discretely move from one clear point to the next. Though actually, discreetly — with ee in the middle — is a different adverb. That one means “quietly, so other people do not notice.” This one is about being apart, not about being secret.