Viscosity is the force of friction that acts between layers of a fluid in contact, thereby inhibiting its motion.
Viscosity is why you see grease flowing slower than petrol.

Uses of viscosity
1. Fluids with a lot of viscosity are used as lubricants to prevent the moving parts of machines from coming into contact with each other.
2. Viscosity is why you water down the pap you prepare for a baby feeding on semi-solid food.