Salvation is Provided through the Provision of God (3:21). 
Genesis 3:21 shows how God met the practical, physical need for providing clothing for Adam and Eve. But the meaning is much deeper than that. It is just as their nakedness (3:7) went much deeper than their physical nakedness. It represented first the innocence, then the shame of Adam and Eve. Likewise, the provision of clothing in verse 21 represents the ultimate provision of salvation by God through Jesus Christ. This verse shows us four things (adapted from James Boice, Genesis [Zondervan], 1:189-192):

• Man needs a covering for his sin. The thought of standing with our sin exposed before the light of God’s presence should be more intolerable than the thought of going stark naked for a job interview at the White House. We all need some sort of covering for our sin.

• Man’s attempts at covering himself are inadequate. Adam and Eve’s fig leaves wouldn’t do. Man often tries the fig leaves of good works to make himself presentable to God, but God cannot accept that. All the good works in the world cannot erase our sin, which is the problem.

(3) Only God can provide the covering we need for our sin. He takes the initiative in properly covering man. He strips off the fig leaves and clothes Adam and Eve with animal skins. Adam and Eve did nothing; God did it all. We cannot receive God’s salvation as long as we offer Him our fig leaves. We must let Him provide everything, as He has in fact done in Christ.
(4) The covering God provided required the death of an innocent substitute. If, as I think we can assume, Adam and Eve witnessed the slaughter of these animals, it must have shocked them. This was the first time they had seen death. As they saw the animals (perhaps lambs?) having their throats slit and writhing in the throes of death, they must have gained a new awareness of the seriousness of their sin and of the greatness of God’s grace in providing for their sin. They learned that without the shedding of blood, there is no adequate covering for sin, but that God would accept the death of an acceptable substitute. Of course the blood of animals cannot take away our sin, but only the blood of Christ, to whom the animals pointed (bible.org).
 
The choice ultimately is this: stand before God in our own power and wear the fig leaves of our creation. Or, we stand before God clothed in the righteousness provided by Jesus Christ his Son.