In the Marvel Universe, there is a clearly defined scale accounting for the power level of the various superheroes, villains, robots, mutants and aliens that bum around, because of course there is. Us geeks need something tangible to hang our terrible arguments about "who would win in a fight" on, which is exactly what this scale does. At the lower end of the spectrum you have the everyday shmoes - the comic book equivalent of us civilians - and at the very top of the scale is where you'll find the so-called "Omega level" superpeople.
A good example of an Omega Level character is the Hulk. Bruce Banner, when he gets a little cheesed off, has a near-unlimited capacity for strength, endurance, stamina, and making trousers purple. For a spell he could actually harness the gamma radiation in his body to make himself eve stronger, meaning he had unlimited potential to cause havoc. Which is why he got dumped out in space by the Illuminati. There are characters even stronger than the likes of the Hulk, though, and they are Omega Level Mutants. All mutant abilities are pretty impressive - well, almost all - but the Omega Levels are different.
They are characters who were born with the X-gene and have had an almost limitless amount of power at their fingertips as soon as they left the womb (or even before, in some cases). They're the ones who wanna watch out for. And they're the ones who dominate this list of the top ten most powerful X-Men of all time, so don't expect to see any Wolverine or Cyclops in here. And definitely not Gambit, mon ami.