An intransitive verb is one which does not admit a direct object. Intransitive verbs typically describe actions which are simply done, or simply happen, without being done, or happening, to anyone or anything. Examples in English are sit, die (although one can die a horrible death, this does not appear to be a proper direct object), and slouch. You might well sit on a chair, but you don't 'sit the chair'. The chair is the indirect object. Equally you can't die something or slouch something.
Oppose transitive verb, where there is also information on differing transitive and intransitive uses of verbs like burn, smoke and drown.