Mekboyz are a type of Ork Oddboy in Warhammer 40,000. They are in charge of technology, making them the Orky equivalent of a Techmarine. Their knowledge is purely instinctual; Mekboyz have no idea how their weapons and vehicles work, they just do. Give a Mek a pile of random mechanical components and he'd subconsciously identify what one of said components is and how it can be used with other bits. This is thanks (to some degree at least) to the Orks' innate pyschic field; so long as the Orks believe the inventions should work, they just will. The extent to which this is true has varied throughout the history of 40k as a game; in earlier editions, this meant literally anything the Orks believed (making Mekboy the easiest job in the universe; just take a piece of metal, make it roughly gun shaped, and watch it work), but more recently this has been toned down to an extent. ((Because obviously Games Workshop cannot leave anything as kickass as banana guns that work in the universe))
Going into a bit more detail your average Ork comes out of the ground with a bit of technical know how. Give him a stick, duct tape and a suitable rock and he'd make a basic choppa. Give him a few tools and a pile of pipes, springs, scrap metal and bullets and he'd bang out a rough slugga. Meks, however get the lion's share of this. They're as a rule less aggressive than most orks. They still love a scrap, make no mistake, but they also love their work. Mekboyz usually find a project and obsess over it. Sometimes they set up a workshop making stuff for other orks for Teef, sometimes they get engrossed in keeping a ship running and sometimes an established mek or Warboss knocks them on the head and gives them a task. Any mek worth his salt has a bunch of grots to do stuff for him.
Their closest counterpart amongst the oddboyz is the Dok, or Painboyz, whose obsession is almost identical, but focused on surgery & anatomy rather than machinery. Because of this, Mekz and Dokz tend to get on pretty good (for Orks, anyway) and this unity is essential to Orky society, since it takes the efforts of both of them to create cyber-implants and to construct Killa Kanz and Deff Dredz.
Though most mekz are more interested in building shit to kill stuff than fighting per say, they do have their own hierarchy. The biggest, strongest and smartest mekz become Big Mekz, who are actually capable of taking over and leading their own Waaagh!s.