'Ned' is a mainly Scottish term meaning a hooligan or otherwise antisocial young man. Researchers found that neds pronounced words differently to youngsters of a similar age who identified with other sub-groups (eg goths, those interested in sport, those more interested in schoolwork). These other groups often had their own distinctive ways of speaking too.
Researcher Dr Robert Lawson from Birmingham City University studied a group of Glasgow lads over three years, and found that they adjusted their pronunciation to identify themselves as members of a particular social group. Neds pronounce the 'a' vowel sounds, as in words like bar or man, lower and more forward in the mouth than other young men.
Here's the full Scotsman article.
Plenty of commentators on the above article have their theories about where the word 'neds' comes from. This BBC article about the forthcoming film Neds, set in 1970s Glasgow, says it stands for 'non-educated delinquents'. That is unlikely. The OED says the origin is uncertain, but that it could be linked to the name commonly given to donkeys -- Neddy.