Te explain what baptism is, we have to look at where it first shows up in the Bible.
In Luke 3, we see a man named John reminding the Israelite people of their need for God. He travels through the surrounding areas of the Jordan River, encouraging his listeners to be "baptized" as a sign of repentance for walking away from God. Being "baptized" simply means the he immersed them in water, bringing them back up as a symbol that they would be a new person from that point forward.
That's essentially what baptism is: a picture of what God does in our lives when we surrender our will to Him.
The church has continued to practice baptism as not only a symbolic act, but as one of its central sacraments. A sacrament is a means of grace - or a way in which God infuses His love into our lives.
We came to Him broken and sinful, but are then made new and full of hope and grace. In the same way, when we enter into the water, we enter as men and women who were broken, lost, and hurting. When we emerge from the water, it represents that we've been washed of our sins, freed from our chains, and made into a brand-new person.
Baptism is a symbol of an incredible work that is going on inside of us.
As we are immersed in water, we are being immersed into a relationship with our Heavenly Father through the work of His Son Jesus Christ (on the Cross), by the power and person of the Holy Spirit. We are being symbolically baptized in the death of Jesus and raised up just like Him from the tomb. Immediately signifying our spiritual rebirth and sealing us for the future when our lives on earth are complete.