The sides of a slope, like a hill or a mountain.
Slopesides are just the parts on the side of a hill or mountain.
Examples:
We hiked up the mountain and sat on the slopesides to have our picnic.
The trees on the slopesides are very tall.
She likes to ski down the slopesides.
The rabbit hops along the slopesides, looking for food.
The slopesides are covered in snow in the winter.
The slopesides are a great place to see the sunset.