Hello There!
Let's first talk about "What Is Friction"
Friction is a force that pulls when two object touch each-other. Friction happens because the molecules on one surface interlock with the molecules on another surface.
Now, let's get back to our original question "What Effect Does Friction Have On A Roller Coaster"
On a roller coaster, friction is a force that opposes motion and significantly slows the cars as they move on the track.