숨다, 숨기다, 감추다 are mean 'hide'

At first 숨기다 is 숨다's causative verb.

For instance

숨다 : 나는 나무 뒤에 숨다 (I hide behid tree)

숨기다 : 가방을 나무 뒤에 숨기다 (Hide a bag behind tree) 

(=> just for memorzie it easy way : ~은/는/이/가 숨다, ~을/를 숨기다.) 

감추다 is almost same as 숨기다 but little bit defferent.

we use '감추다'  hide something because don't want someone can find it.

and '숨기다' just hide something out of sight.

Hope you can understand it even though it is not that perfect explanation :)