An intransitive verb is an action verb (that is, it is neither a linking verb nor an auxiliary verb) which does not have a direct object.
The action is still being done, but it is not being done to anything or anyone else.
Most verbs can be both intransitive and transitive depending on the sentence. The verb to go, however, is always intransitive.
In most dictionaries the abbreviation v.i. means "verb, intransitive."
Transitive: He runs a large corporation.
(The verb runs has a direct object, corporation.)
Intransitive: He runs around the block daily.
(There is no direct object.)