What Are “the Keys of the Kingdom”?

The "Keys" given to Peter, we're not literal keys. The "Keys" indicated that Peter would open up knowledge and opportunities for distinct groups of people to enter into God's Kingdom.
Peter used the first key at Pentecost 33 C.E. When he urged Jews and Jewish proselytes to repent and be baptized.
The second key was used not long after the martyrdom of Stephen. Peter and John laid hands upon recently baptized Samaritans after which these new converts received Holy Spirit .
Peter used the third key in 36 C.E. He extended the hope of Hevenly life to uncircumcised Gentiles.