The other answer is technically correct, but I don't think the term "idiot hair" is actually common in the English-speaking anime community apart from on TVTropes. The Japanese term for this is ahoge, which literally translates to "idiot hair", but that translated term isn't particularly frequently used. A search 'Ahoge anime' gets around 700k Google hits for me, while '"Idiot Hair" anime' only has around 10k hits. Among the translated anime I've seen, some more liberal translators will keep the term "ahoge", while others translate it to other English phrases such as "cowlick". I can't think of any cases where the term "idiot hair" itself was used.
The hair style and the use of the term were both popularized by the gag anime/manga Pani Poni Dash, specifically by the character Katagiri Himeko. Given her personality, the term "idiot hair" is justified in that case. The anime for this aired in 2005, but characters with similar hairstyles have existed for a lot longer.
However, while the hairstyle can signify a childish or idiotic personality, it does not always do so. There is no single correct meaning for it, and it's quite common nowadays for it to be used only for stylistic reasons, similar to the strange colors often used for hair in anime.