How to explain the resistance of a liquid or gas and why they have different viscosities using particle theory

And how does the thickness of a fluid compare to its viscosity? Give an example.

Not all the particles in a fluid flow (current) are moving with the same speed.Those particles next to a solid surface are often moving with the same speed as the surface (due to the intermolecular forces of attraction between them) .. whereas other particles away from the solid are moving at a different speed.
Water particles on a river bed, or against the sides of a pipe are at rest whilst others away from these positions are moving. When we walk through still water, those particles of water against us are moving with us whilst others further away are at rest. Either way a resistance is felt as neighbouring 'layers' of particles shear against each other as they move at different speeds. There is a resistive force associated with this shearing as the particles have to break intermolecular bonds to move past each other at different speeds - fluid viscosity is caused by this resistive force (and is a measure of it)

Syrup and thick oils pour slowly because of the relatively large intermolecular forces within these fluids - we refer to them as viscous liquids. Having strong intermolecular forces makes a fluid 'thick' in that it's particles don't separate easily to enable it to flow quickly.

Basically the kinematic viscosity η = μ / ρ (μ is the normal viscosity, ρ is the density) has units m²/s, which are the units of a diffusion coefficient. They represent how good a substance is at diffusing momentum. If you purse your lips and blow a jet of air at an outstretched hand, you can feel that the effect gets more diffuse at further distances; that's what I mean by 'how good it is at diffusing momentum.' Diffusion in general follows an equation like:

∂c/∂t = D ∂²c/∂x²

...and so no matter what the units are of c, the units of D are m²/s and it describes the sorts of distance/time scales that this thing diffuses over.