Before you start pulling on the object, there is only the gravitational force pulling down which is balanced by the normal force exerted by the ground. At this point, there is zero friction force. As you slowly pull the object laterally, a friction force arises which acts in the opposite direction as the pulling force. As you pull harder on the object, in order for the object to remain motionless, the friction force has to also increase. However, there is a maximum friction force F_f which is known to be equal to F_f = _s*F_N where _s is the coefficient of static friction and F_N is the normal force. Both the coefficient of static friction and F_N are constant values which do not change depending on how hard you pull. This means that once the pulling force exceeds this quantity of _s*F_N, then there will be a net force acting on the object and from Newton's 2nd law, this will cause the object to start moving.