Baptism is a Christian rite or ceremony that is commanded or instructed to be done when a person becomes a believer. Biblical baptism means momentarily dipping or immersing a person in water. The picture is of a person dying, being buried and then rising from the grave and coming back to life; in two simple words death and resurrection. People are not buried by sprinkling some dirt on a person's head and aspersion is not a practice found in the Bible.
This topic is covered extensively on Wikipedia and elsewhere. I don't think it necessary to review every detail. The meaning and intent of the act seem clear and simple to me.
Some organizations and denominations make this into an initiation or rite of passage and make it a requirement of the organization to become a member. The specification and details of the process seem more elaborate than what the Bible requires or specifies. It seems to me that the important thing is belief and trust in God's mercy and grace. Then do it! Clear and simple.
The symbolism in baptism includes the following points.
What Adventists Believe About Baptism
Water Baptism (Stephen Wellum)
Baptism and Christian Initiation (Catholic)