If something is sandier, it has more sand on or in it.
Sandier can make people think of beaches, deserts, or building sandcastles.
Examples:
The beach is sandier than the park.
I like the sandier part of the beach.
The sandier it is, the better for building sandcastles.
I don't want to go to the sandier beach, I like the grass.
The desert is even sandier than the beach!
The sandier the beach, the more fun we have.