When things rub together they can slow down.  This is called friction.  

Something can move quickly or slowly.  Something moving more slowly is called going slower.

Things can be smooth.  Things can have parts that stick out.  Gym shows have parts that stick out.  These are called bumps.  Bumps can add friction.  Our gym shoes help to slow down by making friction.

The bumpy part on the bottom of a gym show is called a tread.  The bumpy part of a tire is called a tread.  A tread helps to slow somthing down.  It makes friction.  Bumps create friction.