The verb "essere" means "to be" in Italian. It is even more important than the verb "to be" in English, since it also serves as the auxiliary (helping) verb for the perfect tense of 33 key verbs in Italian. In other words, to express past action with the following common verbs and with reflexive verbs, you don't use avere ("to have") -- as you would in English ("has gone", "have come") -- you use "essere":
andare (to go), venire (to come), entrare (to enter), uscire (to go out), arrivare (to arrive), partire (to go out), torare (to come back), stare (to stay), esistere (to exist), essere (to be), diventare (to become), sparire (to disappear), costare (to cost), dipendere (to depend), succedere (to happen), morire (to die), nascere (to be born)
"Essere" is also used for telling time (Sono le otto = It is 8 o'clock) and origin (Siamo di Firenze = We are from Florence). Below are tenses of "essere" in the indicative, conditional, imperative and subjunctive moods. Note that essere is an irregular verb. For your convenience, the moods are color coded. The indicative tenses, for instance, are shown in green.