Also for Adam and his wife the LORD God made tunics of skin, and clothed them. [Genesis 3:21]
The first sin of the human race was committed when Adam and Eve ate the fruit that they were not supposed to eat. Realizing they were naked, they tried to cover their bodies with fig leaves. But that was apparently not good enough. The LORD used animal skin to make tunics for them. An animal was sacrificed for this purpose.
Like Adam and Eve, many people try to cover their sins with their methods, but it is not good enough. The LORD has His plan for salvation for mankind, and that plan is Jesus. That animal did not sin but was sacrificed for Adam and Eve, just like Jesus had no sin but died on the cross, covering us with His blood so that we can stand before the LORD.