It means that fighting her isn't really a test of your skills, it's just checking to see who has higher stats. This complaint is often thrown at characters that lack skillshots or mechanics that require much choice on the part of the person playing them.
Kayle for instance, right clicks you and presses Q, then E. Neither can miss and then who wins is based on if she has the stats from her items to beat you not any skill on her part. Oh sure, her W and R play a factor, but barely one... since they're BOTH also impossible to miss and they're just used when needed. The ONLY skill involved in kayle winning a 1v1 is if she predicts the enemy well enough to ult as they use a high damage spell or something. Therefore, she is primarily a test of your stats, not your skill. She is checking your stats, if you will. A statcheck.
Granted, the enemy can still outplay KAYLE if they have a more versatile kit (they will), but Kayle herself has no real options beyond "Q then E, W and R if needed" in every fight. None can miss and the only ways she can "outplay" you involve mechanics outside her kit... like juking into bushes to drop line of sight or flashing a wall or something.
Does that explain it?