Ocean acidification is when carbon dioxide from the atmosphere is dissolving into the seawater. Since this combination is happening, a chemical reaction is occuring between the seawater and the carbon dioxide which is producing carbonic acid.
What is Carbonic Acid?
Carbonic Acid is an acid which is made of several elements through chemical reactions. It is also known as H2CO3. It is the reaction product of the combination of water and carbon dioxide. The main way the chemical is known is it's part in ocean acidification. But, it is in other places too. It is found in many soft drinks and constitutes marble and chalk once it is added with lime.
How is Carbonic Acid dangering the enviroment?
Carbonic Acid is bad for the environment because it causes the water acidity to become much higher than the oraganisms that live in the ocean can handle. But, this mainly matters for two main reasons which are that coral reefs are not able to sustain themselves and shellfish/shells are not able to build their shells without them dissolving due to the acidity of the ocean. This is also bad since coral reefs are the habitats to many other usually smaller fish, leaving them shelterless and much more open as prey.
Here, a chart can be seen of the amount of carbon dioxide in the ocean over many years. The CO2 in the atmosphere increases at almost the same rate at which the ocean is gaining CO2 and becoming more acidic.
Next, four pictures over a period of time can be seen of how carbon dioxide can wear away at shells/other shell animals. This is caused by the fact that the normal calcium that shellfish have built in dissolve once mixed with carbon dioxide. Many other examples of animals that are affected inlcude coral, clams, mussels, sea urchins, barnacles, etc.
Lastly, here is a more scientific explanation of the reaction between water and carbon dioxide.