Have you ever read a manga, or anything fictional for that matter, where the main character is just so ridiculously strong or perfect that nobody could ever really compete? And it wasn’t handled well, either. The character is just so untouchable that there’s entirely no way to relate to him or her.
This is unfortunately a problem for many manga nowadays, because it’s simply easier to write a perfect character than to write an actually complex one.
These characters are called Mary Sues (the term for men is Gary Stu, although some people just use Mary Sue for all genders). Some examples are Akagi Shigeru from “Akagi”, Medaka from “Medaka Box” (although it is a parody) or Kirito from “Sword Art Online”.
From “Sakamoto desu ga?” by Sano Nami.
Please note that a character just being strong is not enough for it to be considered a Mary Sue. There are great examples of extremely powerful characters given clear flaws that make them not perfect, such as the “high-rankers” in Tower of God. They are supremely powerful beings, almost gods in comparison to the main character, but they are just regular humans, personality-wise.
The most important symptom that Mary Sues exhibit is a sense of self-insertion and wish-fulfillment. Does it feel like the character is too perfect, or an idealized version of a human being? It’s very possible that the mangaka is self-inserting and showing you how he wishes he could be. This makes for boring stories!