— There are three modes for a chosen trait in in this mod: Required, Pool and Exclude.

• Required means it will search for a pawn with this trait;
  
• Pool means that these traits are preferred, but are totally optional, so a pawn without those is acceptable;
  
• Exclude means that any pawn with that trait is unacceptable.

You can change these modes by clicking small arrow icon to the right of the trait, each trait changes individualy.

"Required traits in pool" refers to the second tag, this window lets you choose how many of traits in the optional pool need to generate for the pawn to be considered acceptable. So 0 means, that none of them are required, 1 means only one is, and so on.

This way you can add a little randomisation to generation, while still having control over what kind of pawn you want to generate.

You want to generate an op pawn for Naked Brutality playthrough, so you decide to choose appropriate traits.

• You add "Super-immune", and choose "Required" (option by default). This trait is 100% required for a succesful generation, without it, the pawn is automatically skipped.
  
• You add "Fast Learner", "Too Smart", "Trigger-happy", "Jogger" and choose "Pool". Now, these options are in a pool and now considered preferrable by the mod.
  In "Required stats in pool" you pick "1".

Now, the mod will generate pawn until in finds one that has "Super-immune" and at least one of the traits in pool.
e.g.
1. A pawn with "Super-immune", "Fast Learner" and "Iron-willed" (which you didn't choose);
2. A pawn with "Super-immune", "Trigger-happy" and "Jogger" (Both of which you chose);
3. A pawn with "Super-immune" and "Too smart" (Which you chose, the pawn generated with only two traits):
— All of these pawns would be considered acceptable by the mod, according to your filters.

What happened with your case is: you didn't have any traits in optional pool, only in required, so, when you check "Required traits in pool" to "1", it couldn't find a suitable pawn.

— You picked more than three "Required" traits, or picked three "Required" and put one in the "Pool", and checked "1" in "Required traits in pool".
— Or you checked "4" in "Required traits in pool", even if you have 4+ traits in pool.
The reason for these fails is that Rimworld can only naturaly generate pawns with up to 3 *usual* traits, so you are filtering for more traits than is possible. The fourth trait can generate, but it is *always* reserved for pawn's sexuality.
— Alternatively, you chose "Required" traits, that can't generate together, like "Too smart" and "Iron-willed".

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Now, my personal recomendation is to not use trait filters at all, and just add them with character creator mod, as long as you only have three (or four, with sexuality) traits in the end, it should be vanilla-friendly, and possible with game's generation. This mod, however, is great for generating skills and passions, so you can have a strong pawn, yet still have numbers that are possible with normal generation. Then, you can just slap wanted traits using character mod and get a "legally-op" pawn, that you could've technically gotten with normal generation, if you got really lucky.