What does an altar symbolize in a christian church? ?

The altar can represent two things. Firstly, it can represent Christ's offering by his death on the cross. It can also symbolize the fellowship table at the last supper with Jesus and his disciples at the Passover, in which Jesus said "Do this in remembrance of me". 

The altar also symbolizes the presence of God. It reminds us of the sacrifice of Christ, as said in the last comment. Unlike in the Old Testament, we don't need to give blood offerings to receive forgiveness, since Jesus died once for all sins, therefore taking over the purpose of altars, hence the symbolism.