I know "się" is the only reflexive personal pronoun..but

What does the use of "się" in "jak się masz?" do?

completes a sentence? 'jak masz?' only would beg for a 'jak co mam?'

oh wow of all the complicated Polish grammar...I can't believe I was confused with THAT! ale dzięki za pomoć:)

"się" stands for yourself in this case , like 'you' in - How are you

that's it.. yourself.

In fact, the Polish word się is a reflexive pronoun stolen from the Russian Language and then just shortened and simplified. In Russian the word is cjebja. There are many examples of places where the Poles, not having such a word in their own language, simply 'finds' a word in someone else's!

What's your source for saying this? Seems to me it's more likely something both languages got from Old Slavonic.

In fact you have no idea about facts.
First, the reflexive pronoun "się" is of protoindoeuropean origin much like spanish "se", English "self", German "sich" or Swedish "sig".

Second, there is no simplification of any sort in the case of Polish "się", as it relates to russian suffix "-sya" or -ся/-сь (for instance vernulsya).

With the complete ignorance, malice, and stupidity, typical of your "linguistics" drivels you don't even realize that the example you give "cjebja" translates into Polish also as "siebie".

Jeez! Full contact linguistics on this forum.

Jak się masz?
How oneself to be?

this is the nearest I can get to a 'direct' translation to English.
As a beginner I am finding the lack of dirct translations quite complicating to the whole process. I'm hoping it's just a bottom heavy subject and that, as things start to fall into place, the learning momentum will increase.

I don't agree with that. It hasn't been mentioned that the word "się" has other uses too than just the reflexive. Sometimes it is used to make impersonal constructs such as:

tak się robi - that's how it's done

and sometimes it simply changes the meaning of a verb as in this case:

mieć - to have
mieć się - to be, to feel

so the literal translation to English of "jak się masz" is either "how do you feel" or "how are you".

I'm learning English grammar as a side effect of Polish so I'm unsure as to what you mean by 'reflexive'. ...but I'm here to learn, if you feel like explaining?

Anyway, I reached the literal translation of 'Jak się masz' by finding the dictionary definition of 'Jak' (how), 'się' (oneself) and 'masz' (form of 'miec' - to have).

By this method the nearest literal translation would be:
How oneself to have?
(I incorrectly used the verb BYĆ [to be], rather than 'MIEC' [to have] above.

Sure, reflexive is just in English words like myself, yourself, herself, themselves etc..
In Polish for once things are simpler than in English - you just use "się".
OK I lied, it's not totally that simple - you also sometimes use "siebie" and "sobie".
"siebie" is the emphatic form which follows prepositions (compare with cię and ciebie) and sobie is the dative form (off the top of my head - I might be wrong).
While we're at it - sometimes you will here "se" in slang and spoken Polish, e.g. kup se samochód - buy yerself a car

You were close - you looked up "mieć" in the dictionary but you should have looked up "mieć się" which is a different verb as per my earlier post.