It is interesting that Peter assumes everyone knows what baptism is. This is because it is not a concept that first needs to be explained. Even in the time of John the Baptist it was a familiar practice with the Pharisees questioning the reason for the baptism and not the meaning or nature of the practice. The reason for this familiarity is that baptisms or ritual washings were a common part of Jewish religious experience. In the Old Testament there are various laws which stipulate when either a priest or a layperson was required to wash themselves. Typically it was whenever someone became unclean as a result of contact with an animal or sickness or even just due to the normal habits of daily living. Before you can meet with God you had to be washed, a visual symbol to confirm the need for cleansing if you are to come into God’s presence. So everyone understood that already: baptism is associated with cleansing.
38 And Peter said to them, “Repent and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins, and you will receive the gift of the Holy Spirit.