what part of speech is it for the word "prior to"?
I read this word several times but I'm not familiar with it. Does it just simply express the meaning of something happening before something else? Why don't we simply use the word 'before'? Is it because with the word 'prior to', we have to follow by a noun or noun clause. But with the word 'before', we could either use "subject + verb + object (optional) or a noun?

e.g.
I expected you should have prepared everything prior to my arrival.
I expected you should have prepared everything before I arrived.

Question

what part of speech is it for the word "prior to"?
I read this word several times but I'm not familiar with it.  Does it just simply express the meaning of something happening before something else? Why don't we simply use the word 'before'? Is it because with the word 'prior to', we have to follow by a noun or noun clause.  But with the word 'before', we could either use "subject + verb + object (optional) or a noun?

e.g. 
I expected you should have prepared everything prior to my arrival.  
I expected you should have prepared everything before I arrived.

Answer

"Prior to" means "before" or "earlier than".
All ot these are correct and acceptable.
The only reason to use "prior to" is that it is considered (by some) to be more formal.
You could simply use the word "before" and be perfectly correct.
As to why we have so many ways to say this, I can only say that every language has more than one way to express a number of concepts.  English has more than some because it is a mix of German and French.

Answer

prior to my arrival
prior to = preposition (of time)