What's the u prefix in a Python string?

You're right, see 3.1.3. Unicode Strings.

It's been the syntax since Python 2.0.

Python 3 made them redundant, as the default string type is Unicode. Versions 3.0 through 3.2 removed them, but they were re-added in 3.3+ for compatibility with Python 2 to aide the 2 to 3 transition.

The u in u'Some String' means that your string is a Unicode string.

Q: I'm in a terrible, awful hurry and I landed here from Google Search. I'm trying to write this data to a file, I'm getting an error, and I need the dead simplest, probably flawed, solution this second.

A: You should really read Joel's Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!) essay on character sets.

Q: sry no time code pls

A: Fine. try str('Some String') or 'Some String'.encode('ascii', 'ignore'). But you should really read some of the answers and discussion on Converting a Unicode string and this excellent, excellent, primer on character encoding.

My guess is that it indicates "Unicode", is it correct?

Yes.

If so, since when is it available?

Python 2.x.

In Python 3.x the strings use Unicode by default and there's no need for the u prefix. Note: in Python 3.0-3.2, the u is a syntax error. In Python 3.3+ it's legal again to make it easier to write 2/3 compatible apps.