If a frictional force is preventing an object from moving, it is called a static friction force.  The maximum value that the static friction force can have is given by

• Ffs = ms*Fn

where Ffs is the magnitude of the static friction force, Fn is the normal force on the object due to the surface, and mk is the coefficient of static friction.  For instance, suppose a rocket sled is resting on sand, and the thrust is gradually increased.  At first, the applied force of the thrust is balanced by the static friction force, and the sled does not move.  As the thrust gradually increases, the static friction force also increases, and the sled still does not move.  However, just after the static friction force reaches the value given by the equation above, the sled starts to move.  The static friction force has reached its maximum, and it can not longer increase as the thrust increases.  Thus, there is a net force on the sled and it begins to accelerate, in accordance with Newton's Second law.  The force of static friction can have any value from zero to its maximum value, whatever is necessary to keep the object from slipping.