when adam sinned by disobeying god in the garden after being prompted by his missus, and after both denied any wrong doing or knowledge of it to gods questions, they both (in particular adam since through one man sin entered the world) were cast out of the garden and it was only then that they realised that they were naked (which is indicative, imho, of them becoming aware that they had indeed sinned)
since they were now naked, in more than one sense of the word, and since they had sinned, the offence was required to be covered. and the only way sin can be covered is by the shed blood of a sacrifice.
so god, in his wisdom, slew an animal to allow both adam and eve to cover themselves and hide their nakedness and also to cover their sin.
this is the precursor of the later sacrifices to cover personal and national sin which led to the ultimate sacrifice of jesus in which his shed blood is meant to cover the sin of mankind.
