The heavens opening
Mark uses the old testement symbol of the "heavens being torn apart" to symbolise the presence of God in Jesus' Baptism. Many old testemant biblical references shows that God was often asked to open the heavens (the book of Isaiah).
The Holy Spirit descending on Jesus is another symbolic reference which was also often used in the old testement thus giving it an important image.
Genesis 1:2 record the "Spirit of God to be brooding on the waters like a dove"
As a dove is often interpreted as a sign of peace, many chirstians think of this symbolically to mean that Jesus experianced the peace of God in his heart
To clarify, there was no dove in the actual story : "like a dove"
The Voice of God from heavan
The voice of God coming from heaven shows that Jesus was receiving direct divine approval and confirmation that Jesus was God's Son.
" You are my son, whom I love; with you I am well pleased". This shows that God approved of what Jesus was doing