This is the 10002nd most frequent Japanese word.
転がす
"転がす" means "to roll (something)" or "to tumble (something)."
Here, '転がす' is used to denote the action of rolling an object, specifically a ball, on the ground.
ボールを床の上で転がす。
I roll the ball on the floor.
In this figurative usage, '転がす' conveys the action of considering or playing around with an idea mentally.
アイデアを頭の中で転がしてみる。
I roll the idea in my mind.
This application of '転がす' describes the action of physically moving a heavy object, like a stone, by rolling it.