Rentrer can be used with avoir or être in Le Passé Composé... and changes meaning

Most verbs use either avoir or être as the auxiliary verb in Passé composé (or other compound tense)but rentrer uses both, depending on its grammatical usage* and what it means in the sentence.
 
*Grammaphile's Corner : the technical grammatical distinction between these cases is actually whether the verb is used in a transitive or intransitive manner. 
- The transitive version (the version with a direct object) uses avoir.
- The intransitive version (lacking a direct object), uses être.
 

être + rentré [quelque part]

= to go/come in(to) [something/somewhere] 
= to go/come back in(to) [something/somewhere]
= to go/come/get home

Je suis rentré de vacances tard hier soir.I came back from holiday late yesterday evening.

Vous êtes rentrés dans la pharmacie à six heures.You came into the pharmacy at six o'clock.

Quand tu es rentré dans la pièce, elle n'était plus là.When you came back into the room, she was gone.

Note that in each case where être is the auxilliary, the verb rentrer is followed by a preposition (en, sur, dans, à, etc.)
So in these cases rentrer is usually about going or coming back in or into, going or coming home, going or coming in or into.

(See also Agreeing past participle with subject's gender and number with (+ être) verbs in Le Passé Composé)

avoir + rentré [quelque chose]

= to take/bring/get [something] back inside

J'ai rentré le linge une fois qu'il était sec.I took the laundry back inside once it was dry.

Vous avez rentré les meubles de jardin à cause de la pluie.You brought the garden furniture inside because of the rain.

When rentrer is followed immediately by a noun (as opposed to a preposition), it uses avoir as the auxiliary, like most verbs.  
 
It can be very tricky to get the distinction here if you think in terms of what rentrer means in English. English verbs are very often 'prepositional', meaning we say things like to go back into a house as well as re-enter a house which are equivalent in meaning but grammatically very different - English verbs very often have prepositions where they don't in French!

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Examples and resources

Je suis rentré de vacances tard hier soir.I came back from holiday late yesterday evening.
Vous avez rentré les meubles de jardin à cause de la pluie.You brought the garden furniture inside because of the rain.
J'ai rentré le linge une fois qu'il était sec.I took the laundry back inside once it was dry.
Quand tu es rentré dans la pièce, elle n'était plus là.When you came back into the room, she was gone.
Vous êtes rentrés dans la pharmacie à six heures.You came into the pharmacy at six o'clock.
Clever stuff underway!