Question

why is it difficult to balance our body when we accidently step on a peal of banana

Answer 1

because in that case the base on which we stand become slippery and almost the friction also becomes zero

Answer 2

When we walk our foot pressurises the ground in downward direction and to counteract that force, ground also pushes our foot in upward direction (with same force) and with the help of initial angle between our foot and ground, our foot tries to push back ground in backward direction and hence we're able to walk.