When you stand on the ground the ground will exert a vertical force (W) on you that just balances your weight (Figure 1). This force is called the normal reaction. It is possible to stand on a surface where there is virtually no friction such as on the ice of a skating rink.
However once you try to walk or run some friction is essential (Figure 2). The vertical force is now G, this is a bit less than W and the frictional force is shown as F. This frictional force acts in the opposite direction to the horizontal component of the force of your foot on the ground. That means when your foot is ‘pushing forward’ at the start of a stride (Figure 2(a)) the frictional force acts backwards to prevent you slipping while when you are ‘pushing backwards’ at the end of the stride it acts forwards (Figure 2(c)).