What does the name hikaru mean?

That depends entirely on what characters are used to write it. For example, written 春姫 (as in Kamisaka Haruhi from the game and anime Happiness!) it means 'spring' and 'princess', but written 晴日 (as in singer Aiso Haruhi) it means 'clear weather' and 'day' or 'sun'. It's also commonly written in hiragana (as はるひ) or katakana (as ハルヒ), which is phonetic spelling; the characters have no meaning.
Since I suspect it might be why you're asking, both Suzumiya Haruhi's and Fujioka Haruhi's (Ouran High School Host Club) given names are written in katakana.