"Come wiff me, and you'll be, in a wurld of pure imajinayshun."
- – Wille Wompya, Ork Mekboy
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 largely instinctual, ingrained at the very genetic level; Mekboyz often have no idea how their weapons and vehicles work, and would be at a loss to explain the underlying principles of their function. Yet, give a Mek a pile of random mechanical components and he'll subconsciously identify what each component is and how it can be used with other bits. This would be impressive enough, but the Ork Mek has yet another advantage that the technicians of other races don't. He believes that his inventions will work, and because of the innate passive psychic field that all Orks possess, they do work (most of the time, at least).
The extent to which this is true has varied throughout the history of 40k's in-game lore. In earlier editions, if you convinced an Ork that a pipe was a gun, he'd use it to fire bullets out of thin air. However, GW eventually realized that this was silly even by Ork standards, so this has been toned down to a great extent. In current lore, if you hand an Ork a slugga that has been stripped of all internal mechanisms and ammo, it won't suddenly start firing bullets just because the Ork believes it should. However, if a crudely manufactured slugga consistently misfires in the hands of a human but *does* fire reliably when wielded by an Ork, it is definitely Orkish belief that is bridging that gap. In other words, technologies that should be impossible become merely unlikely when crafted by an Ork.
As mentioned earlier, every Ork comes out of the ground with a bit of basic knowledge and technical know-how. Meks, however, get the lion's share of this. From a very early age their heads are filled with visions of weapons and fantastical technologies that they don't understand. And where a human Tech-Priest might have no idea where to even start, a Mekboy just starts building. Mekboyz usually find a project and obsess over it. Sometimes they set up a Workshop making stuff for other Orks for Teef with their trusty Mek's Fixa. Sometimes they get engrossed in keeping a ship running. Sometimes an established Mek or Warboss knocks them on the head and orders them to build a war machine of one type or another.
Another important point to mention is that while Ork technical knowledge is primarily instinctual, it isn't exclusively so. Orks can and do learn, refining and personalizing their techniques over time. This is true for Meks as well. Therefore while Ork tech always has a certain one-off, ramshackle quality to it, the greatest Ork mekboyz are capable of producing inventions that equal and sometimes even exceed anything that the other major races have to offer.
The Mek's closest counterpart amongst the oddboyz is the Dok, or Painboy, 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). 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. Meks also work closely with Runtherds, bartering with them for the smartest runts. Any Mek worth his salt has a bunch of Grots to do the fiddly stuff for him, plus some Oily Squigs to lubricate and fuel his contraptions as well as an occasional Paint Squig or two for decoration.
While most meks use the majority of their time to build shit capable of killing stuff, they enjoy fightin' just as much as any Ork. After all, what's the point of making the biggest guns if you don't show 'em off? In fact, the biggest, strongest and smartest Meks become Big Meks, who are actually capable of taking over and leading entire warbands.