What are the antigens on the surface of viruses?
The simple answer is that the antigens on the surface of viruses are simply proteins however what they do can be rather complex.
Cells also have these surface proteins which are very helpful for a variety of tasks. These surface proteins act similarly to a "key and keyhole". The proteins on the cells are the key holes. Then, whenever an organism with the matching "key" comes into contact with the cell, its allowed passage through the cellular membrane.
From there the virus does its usual stuff, replicating its genome and assembling other "bodies" with your cell organelles until it eventually kills your cell and moves on.
This is why viruses can be so deadly and hard to stop. The immune system has the ability to eventually recognize what proteins the virus is using and eradicate it from the body but since the virus is able to infect and replicate so quickly, the immune system usually lags behind at first.