what does seed +x do

[Kanjejou]

what does gaining bigger numpered seed do?

[Iyasenu]

For crops, it upgrades the seed with some sort of additional effect.
For edible crops, these can be either a Potency upgrade for one of the stats (i.e. [Flaky] Lv.1 WIL Potency) or, can be [Rich Collagen] Lv. 4 CHA.
Also, they can get Quality traits such as [Fair Quality] Lv.3 Item Quality, in addition to one of the above effects.
For trees, their fruits will get both of the traits, but the wood will only get the Quality trait.
You can keep growing and regrowing the same crop to keep leveling it up, just by using your sickle when it gets fully grown again.

Anyways, these traits get passed on to the resulting crop.
If the crop already naturally has that effect, it gets added together into a stronger effect.

It's random what sort of stat you get on a seed, so if you want a specific one, you'll have to go find new seeds, grow them, and reap them again with your sickle and hope for the best.

Also, the +X only goes as high as your Farming skill level.
So if you're level 20 Farming, the best you can get is +20 seeds, until you increase your Farming level.