what are skins

Trade offer me here: https://steamcommunity.com/tradeoffer/new/?partner=84957993&token=B0_S3eix

Knife history:
Huntsman Knife| Vanilla
M9 Bayonet | Stained (MW)
Bayonet | Case Hardened (BS)
Butterfly Knife | Urban Masked (MW)
Bayonet | Blue Steel (BS)
Butterfly Knife | Stained (BS)
Bayonet | Stained (BS)
Butterfly Knife | Boreal Forest (FT)
Huntsman Knife | Case Hardened (BS)
Huntsman Knife | Night (WW)
Huntsman Knife | Night (BS)
Huntsman Knife | Forest DDPAT (FT)