The Ork Mega-Dread is a very large walker vehicle. This monstrosity of a walker is to Deff Dreds and Killa Kans in the same way the Contemptor Dreadnought and its offshoots are to Space Marine Dreadnoughts. Thus these things are actually rare, since constructing one requires a Mekboy of both uncommon talent, and high levels of Orkishness, so a Guardsman won't be seeing one on every battlefield (nevermind that a Guardsman won't be seeing many battlefields).
Mega-Dreads are simply more mobile alternatives to Ork Stompas due to having incredible hydraulic power and a compact frame for something of its size and construction. Due to their complexity, Mega-Dread pilots have a whole array of sophisticated (for Orks) implants rammed up their heads in order to use the unique hybrid system that allows them to manoeuvre these oversized tin cans. Combine it with Kill Kannons, claws, a gamut of other high caliber guns and their durability, the Mega-Dreads become something every Warboss would go bankrupt on teef for to acquire for themselves.
To the Mekboyz that are able to build these monstrosities, they are the ultimate killing machines and a Warboss can expect to pay a hefty amount of teef to acquire them.
In 7th Edition, Mega-Dreads are a dedicated close combat shock unit, but boy oh boy does it do its job well. The Rippa Klaws wreck everything in close combat, and get Wreckers stratagem in Cities of Death. The Killkannon, normally too short-ranged to be effective, finds its home here. This thing will blast apart your enemy squads, before the Dread will munch them to bits in close combat. It is also very hard to stop, being armor 13 everywhere but the back (which is 11) and has base strength 10 making for a tasty HoW.
Careful around prolonged Termie throwdowns, you never know what they wield. This guy can also get a Twin-linked Supa-Skorcha that will make MEQs do the burny dance like there's no tomorrow, but the Killkannon can eat Marines at farther ranges. Don't even bother with the Mega-blasta, the Rokkit Launcha has the same Strength (8) and can fire off the same number of shots (one), has the same range (24"), is 5 points cheaper, and does not suffer from the gets Gets Hot! rule. The Mega-Dread can also get Grot Riggers (handy), an additional Big Shoota for Snap Fire, and a Mega Charga (helpful until inevitable tard-out). Best against Marine Equivalents and Light Vehicles/Walkers.
An alternative version of these are the Meka-Dreads. An even more advanced version of the Mega-Dread that has a Mekboy piloting it instead of a normal Ork, Meka-Dreads include a unique kontrol system and swaps its Big Shootas for giant servos that it uses for fixing vehicles.
Unsurprisingly, the Meka Dread Body is also supplied and controlled with a Big Mek pilot, holding the 'complex' Kontrol system of this unique war machine that has been hardwired and jammed into his brain.
Tl;dr, its an upgraded Mega-Dread.
In 8th Edition, Meka-Dreads lacks Mob Rule. It must take either a Kustom Force Field (5++ for all ORKS units entirely within 9"), a Mega Charga (Once per battle increase your Movement by 8" for one Movement phase, but take a mortal wound and can't Advance or Charge that turn on a D6 roll of 1), or Rokkit-bomms (an extra weapon). Keep in mind you have to use the Codex for the KKF point cost.
The Mega Charga and Rokkit-bomms have no points listed anywhere. Also, both the Mega-Charga and KFF are listed as abilities rather than equipment. This makes a Meka-Dread a good choice for a big bubble of KFF that's cheaper than a Morkanaut by 32 points with not much of a decrease in durability and a good companion for other normal Deff Dreads or a Killa Kan wall. Like most heavier vehicles it can fire Heavy weapons on the move with no penalties, and Fall Back without losing the ability to shoot or charge.
Also has an unusual wound ignore mechanic in Ramshackle Monster: It ignores wounds on a 4+, but every time you fail the roll it gets worse by 1 - decreasing to 5+, then 6+ before it stops working altogether after three failed rolls.