In grammar, if you use a verb intransitively, the verb has a subject but no object. In “They arrived,” arrived is used intransitively: we do not arrive something. In “She laughed,” laughed is the same kind of use. Many verbs can go both ways: “He eats” (intransitively — we are not told what) versus “He eats rice” (with an object). Learner dictionaries mark intransitive verbs [I]. This adverb lives almost only in language class and grammar notes; you will rarely need it in ordinary talk. Though actually, “intransitive” here is only a grammar label. It does not mean “not moving from place to place,” and it has nothing to do with public transport.