What does the ending 히 mean?
I mean like the words 완전히, 솔직히, 간신히, etc.

Question

What does the ending 히 mean?
I mean like the words 완전히, 솔직히, 간신히, etc.

Answer

In this case, 히 is used to produce an adverb.

완전히 = totally
간신히 = barely 
솔직히 = frankly

Other ones are 굉장히, 가뿐히...well these two are all I can think of right now.

Answer

let me know whole sentence. 
it uses depending upon the each situation.

Answer

히 can be used after verbs to create either causative or passive verbs.

For example, the causative verb for 입다 (to wear) is 입히다 (to dress).

Lesson on passive voice:
http://www.talktomeinkorean.com/lessons/l6l21/

Lesson on making causative verbs: 
http://www.talktomeinkorean.com/lessons/l7l11/