The coefficient of kinetic friction is the coefficient of proportionality which relates the force of friction which acts on a block which is sliding to the normal force . As it is a coefficient among forces, it lacks units.
You can find out its values for some surfaces by clicking here
It is important to distinguish between this coefficient and the coefficient of static friction m_e. The following expression always applies m_s >m_k.
Study the visual to see how this coefficient varies and to understand its meaning more clearly. Observe how the force of friction varies, until it reaches a maximum value. From this point on, the frictional force is associated with the coefficient of kinetic friction. 

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