Welfarism is the view that what's good (simpliciter) is just welfare, i.e. things being good for various individuals. (Note that this can aggregate: saving two lives is better than just saving one, but there's no larger entity that it is better *for*.)

By contrast, a nihilist might agree that something is "good for" an individual, but deny that this *matters* in any way. That is, they would deny that welfare is good (simpliciter).