Ihr can be three things.
1.) second person plural, nominative case (you all)
Ihr seid meine besten Studenten.
You are my best students.
2.) third person singular female, dative case (to her)
Ich gebe ihr mein Buch.
I give her my book / I give my book to her.
3.) possessive article for masculine and neuter things, third person singular female owner, nominative case (her something) + accusative case (only for neuter things)
Ihr Vater heißt eigentlich Frank.
Her dad is actually called Frank.
Chirlu pointed out in the comment that there are more meanings... so on we go:
4.) possessive article for masculine and neuter things, third person plural onwer, namonative case + accusative case (only for neuter things)
Ihr Professor heißt auch Frank.
Their professor is also called Frank.
4 a.) As with the third person plural in general, this can also be used as a polite "you". It is then written with a capital I.