Verbs can be classified as transitive or intransitive.

Transitive Verb: An action verb that has a direct object.  Trans- means across.  The action is going from the subject to a noun or pronoun that is the direct object of the action verb.

She walked a tightrope.

She walked what?  A tightrope.  Tightrope is the direct object of the verb, walked.  Walked is a transitive verb.

Intransitive Verb: In- means not, thus intransitive means no action going from subject to direct object.  An action verb with no direct object is intransitive.  All verbs of being are intransitive.

She walked down the street.

There is no noun or pronoun that directly follows the action verb, walked, in this sentence.  Down the street is an adverbial prepositional phrase that tells where she walked.  It is not a direct object.

Bill is in room 13.

There is no action here.  Therefore, the verb is intransitive.