The Polite お Prefix

Why is it お名前は何ですか (why the お at the front)?

Good question!

The short answer is that placing an お at the beginning of some nouns will make them more polite, particularly when they relate to or “belong to” the listener, such as the お名前 (“your name”) above.

Similar Examples

Some similar examples of this usage of the prefix would be:

お仕事は何ですか？
  仕事＝しごと
"What is your job?"

お勧めは何ですか？
  勧め＝すすめ
"What do you recommend?"
lit. "What is/are your recommendation(s)?"

The word 仕事 means work/job/business/occupation and 勧め means recomendation/suggestion/advice, but because we’re asking the listener what their job or their recomendation is, we add an お to make things more polite.

Other Examples

Non-“your” Politeness

It should be noted that prefix doesn’t necessarily always have such an immediate sense of “your” to it, as seen in the following example:

お茶はいかがですか？
　茶＝ちゃ
"How about some tea?"
"Would you like some tea?"

This is still a polite question though, and the お preceding the 茶 helps add to that politeness. (That said, it could probably be argued that 茶 can “no longer be used in isolation”, in the same way that the excerpt below explains ごはん.)

Familial Names

You’ll also see the お prefix in familial names, especially where the family member is older/senior:

お母さん
おかあさん
mother

お父さん
おとうさん
father

お姉さん
おねえさん
older sister

お兄さん
おにいさん
older brother

Kunyomi vs. Onyomi, and Other Notes

This お prefix would be written in kanji as 御, and the Wikitionary entry for 御 does a good job of touching on some other helpful points:

This prefix is frequently written in hiragana, both because it is complicated and common, and because its reading may be ambiguous if the term is unfamiliar. When the prefix is read as o- (as opposed to go-), it is almost exclusively written in hiragana.

While in general this prefix is optional, in many cases it is so commonly used that the base word can no longer be used in isolation, as in 御飯 (ごはん, gohan, rice) – the form ×飯(はん) is not used alone, though it can be used as parts of compounds (such as 炊飯, suihan), and the character can be read in isolation as meshi.

Usage varies between speakers, situations, and sex – more polite speech, especially by women, features more use of this prefix, while blunt speech, especially by men, uses it less or not at all (words where the prefix has become mandatory are replaced by blunter terms that do not have the prefix). In rare cases a prefixed term has become impolite, as in 御前 (omae, “you (familiar or derogatory)”).

This character is generally read as お (o-) when it is prefixed to words considered to be of Japanese origin, as in 御水 (おみず, omizu, water), and ご (go-) when it is prefixed to words considered to be of Chinese origin, as in 御主人 (ごしゅじん, goshujin, husband). There are numerous exceptions, however, such as お電話. For (non-Chinese) foreign loan words (外来語, gairaigo), this prefix is seldom used, and both お- and ご- may be used, but お- is somewhat preferred, as in おビール (o-bīru) for beer.

I feel like that last paragraph is a tad vague though, so, more concretely:

So while these are written in hiragana almost all of the time, they are both 御 in meaning, and the usage of お / ご is simply a kunyomi / onyomi distinction. …plus exceptions, because language.

So… when do I use this?

I’m not super confident in my polite speech, so I wouldn’t take it from me, but I’m pretty sure you don’t just go around throwing this prefix in front of every noun you come across. I think it’s more a matter of pattern recognition after a lot of input, and just kind of getting used to what words this prefix is used in front of.

For example, I know that if I was thanking a customer for contacting me, I would refer to their call or email (“your contact”) as ご連絡（れんらく）, and if I went to a bar or bar-style restaurant and wanted a water, I’d ask for お冷（ひや）. Why? Because that’s just what you do!