^Friction enables us to walk

In order to run or walk forward we push (P) the ground backward at some angle to vertical.

The reaction of this push can be resolved in components.

The vertical component balances our weight, where as the horizontal component push enables us to walk forward.

For no slip, F_L ≥ P sinθ

or  μ Pcosθ ≥ Psinθ or  μ ≥ tanθ

On a smooth surface we fail to exert enough horizontal push on the ground as a result receive insufficient reaction i.e. the friction between our feet and the ground is greatly reduced, which causes us to slip. We can avoid slip by taking small steps.