This is the 8614th most frequent Japanese word.
放す
"放す" means "to release" or "to let go."
Here, '放す' is used to mean releasing an object or letting go of something.
ボールを地面に放した。
I released the ball onto the ground.
In this sentence, '放す' implies setting something free, specifically letting the bird go.
彼は鳥を空に放した。
He released the bird into the sky.
The usage of '放す' here involves letting go of physical hold, specifically a hand.