why do we slip while stepping on a banana peel?
Ccomadhidhevi has given absolutely correct answer. So, you can refer to it.
Friction supplies the support force which is necessary for a body to walk, run, sit etc.
Friction is necessary for walking because as per Newton's third law of motion, (every action has an equal and opposite reaction), therefore, we can walk only if the ground we are walking on push our feet back with a force. Now, here friction comes into the picture and helps us by providing the necessary reaction force. As per the third law, the ground would definitely push our feet back but if we are walking on a perfectly smooth ground which has no friction our force would simply cancel out the force reverted by the ground and we would fall.
Hope this helps,
@ ccomadhidhevi good answer. You get a thumb up. Keep posting!!