It's exp for your ninjas (not your main, just the 3 other ninjas).
So basically the thing food give u, seeing as how you probably have more food than you can ever use, pretty useless.

oh ok tnx

hey how do players increase their power lvl im @ lvl 75 but my power lvl is only @ 12500

First, you need to do everything everyday, that means Team instance, Strong Approach, convoy/plunder, survival, ninja exam, ninja test, use all stamina on some instance, daily quest, weekly quest, daily events(fox, masuri, sage, GNW), coin trial (and expert/extreme exp trial) and any events of the week.
Then, it's about managing the resources you do get:

magatama: as even as possible between all slots, you should only ever have magatama of two consecutive level(note there is a tiny exception, but it's minor and just confuses people, so don't worry about it). Similar things should be done for the mystery cave runes and summon runes.

coin: you want to donate to group to max group skill, after which you should spend on level up gear and then finally on 8 gates. On 8 gates, try to keep the levels of the chakra even, with purple being highest level(lv up to a specific first) then orange than red, the actual order is a bit complicated but this will do fine.

stamina: your focus should be farming equipment upgrade material that you NEED (as in you have the scrolls for from ninja exam), awakening your most used ninja (usually 4-12), and for any of those most used ninja that you can, farm out their fragment. Don't waste your stamina on farming stuff you don't actually need.

coupon: should be used in events, I'd suggest waiting for lucky board. Or the cave, until you have at least lv 3-4 rune in all slots.

You should try to pass more ninja exam at least once per week at your level. Every extra level you can farm daily helps.

You should be able to pass survival fully, including the breakthrough levels. If you can't, go check/ask in the strategy forum section. Survival enemy is based on your power and not level, so even if you are low power for your level, you should be able to beat them.

That's all off the top of my head...

Another thing to note is do NOT use stamina potions except on Monday when it's double exp for instances :)

I use them on monthly events, I value a medium thread over 45000 exp.

That said, I do try to go for the Monday thing even when doing that, doing part of my "Tuesday" on Monday--after midnight so monthly reset but before 5am before the 2x goes away.

Unless you are close to lv up(have to be very close for this to even matter) and really want it, don't use them on just ANY Monday, instead on a Monday that have an event that drops event item when you sweep and there is no limit to how many can drop, for example character plates.