If something is stretchier, it means it can stretch more easily or further.
Stretchier things are often more comfortable and easier to use.
Examples:
This rubber band is stretchier than that one.
I need a stretchier shirt for exercising.
The stretchier the pants, the better for playing sports.
I wish my arms were stretchier so I could reach the cookies on the top shelf.
A stretchier rubber band could go all the way around the world.
I like stretchier clothes because they feel better when I move.