Smear layer refers to a layer of debris on the inorganic surface of substrate which comprises residual inorganic and organic components. This layer is produced whenever the tooth structure undergoes a preparation with a bur.
Smear layer will fill the orifices of the dentinal tubules, hence forming smear plugs. These smear plugs decrease dentin permeability by 90% and the smear plug alone can prevent adhesive resin penetration into dentinal tubules. The thickness of smear layer can range from 0.5-2 µmeter and for the smear plug, 1 to 10 µmeter.
Smear layer poses some threat for optimal bonding to occur. That is why it needs to be removed. For example, smear layer needs to be removed prior to bonding by etch-and-rinse (total etch) adhesives.  This will lead to thicker hybrid layer and long, denser resin tags which results in better bond strength.