The coast represents the end of the road literally and also the physical end of the man.

By bringing his son to the coast, he saves him by delivering him to other people who will presumably protect him. His singular goal is to save his son from the aftermath of the disaster that destroyed their world.

We do not know why the coast is an important destination, but the father and son put all their energy into reaching it. It is always clear to them that the coast is where they should be going.

Once they reach the coast, the father dies. He has reached his goal, and there is nothing left for him to do. Once the father dies, the boy can move on. The coast is the end of the journey and the end of the father’s life.

I think that this question might be asking what the coast represents figuratively and literally to the two protagonists in McCarthy's The Road. While those are two different things, they are closely linked. The coast, as a physical destination, offers the possibility of literal safety. This safety is created in a variety of ways. The Man and his son are open to attack and robbery from any direction while on the road, but being near the coast offers them safety from attack from the direction of the water. That could work against them by boxing them in, but the protection factor is still there. Additionally, the coast offers a real chance for food and water. People have been using the oceans for food sources for a very long time. We don't know what happened in the world in this book, but the Man probably thinks that there is a chance that sea creatures were not affected. Being near the coast also means more moisture through increased humidity levels. While they can't drink ocean water, they can harvest moisture from the marine layer that is common along coastlines. These physical advantages of the coast lead to the figurative and symbolic meaning of the coast. The coast offers hope. There is hope that there is more food. There is hope that they will be safer, and there is even hope that the worldwide blight/pandemic didn't affect the coastal regions. There is hope that fellow "good guy" survivors can be found by the coast.

The coast is the ultimate goal of the journey for the Man and his son, where the road is supposed to lead them.  It's not completely clear why getting to the coast is of such paramount importance literally, except the Man briefly mentions following the coast south to try and find others like them.

I would say the ocean represents beauty and hope to both of them.  The last beautiful thing the Man can remember, perhaps, and something the boy has never seen.  To reach it is to make it out of the cannibalistic nightmare of tragedy and holocaust they have survived thus far.

It could also be that the coast is simply a destination, any destination that is not here, not now, and therefore must be better.  McCarthy pretty perfectly illustrates the death of hope when the ocean is not beautiful at all, and the Man ultimately dies there.