An intransitive verb is one which involves an action that is not performed on anyone or anything, i.e., it has no direct object.
Mel walks for miles.
(As "walks" is not being done to anything, this verb is intransitive. "Mel walks the dog for miles" does have an object, the dog, and therefore "walk" is transitive in the second example.)
Every single person voted. ("voted" - not done to anything - intransitive)
The crowd demonstrated outside the theatre. ("demonstrated" - not done to anything - intransitive)
Many verbs can be used intransitively or transitively:
The apes played in the woods. (intransitive)
The apes played hide and seek in the woods. (transitive)
Glossary of grammatical terms
Click on the one with an intransitive verb: