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

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