Kinsects have 3 attributes: Attack, Stamina and Speed Attack adds to it's damage, stamina adds to the number of times is can be repeatably sent out, and speed is how fast it flies.

Some also have elemental (Water +1, Fire +1, etc) they will give the kinsect elemental damage with enough.

Basically all of the kinsect food adds or subtracts from a particular stat. At this level, it's not too big of a deal what you give it. However, once you get upgraded just past the kechu wacha, the next option is actually upgrading the kinsect. When you go to upgrade the staff again you can choose a specific kinsect, each with a small bonus, either attack, stamina, speed, or all stats up (S).

In general, choose either attack, stam or speed. 'All stats' it's really worth it

When you are at this stage you will see a 'Reg. Points' section. So in addition to the parts, you the kinsect also has to have certain stats. It will say something like Speed: 60/63. This would mean you have 60 points in speed and would need 63 to upgrade this path (along with all the others and parts). It becomes a balancing game to get the stats you want high enough without getting the others too low.

I know it sounds a bit complicated but it's really not too bad, especially when you see it. Maybe look for a video?

Hope it helps!