To your questions, what I understand is:
"Is the production how many are produced each turn?"
Yes, under de column "Prod"
"And are the pln the planes that there are?"
No. It means something like "planned production". So in the image, the Bf109G6 is producing each day 30 planes, but there are a "planned production" of 32, so probably there are 2 factories offline right now.
"And stk does this mean how much can be used and if i can use them how do i do that?"
No, stk means "stock". In the image, i.e. there are 100 BMW801D-2 engines in stock.
Yes you can manage the stock changing the production. I mean, if you have 1000 Bf109 parts in stock, you can change some factories to other plane type, and you'll use the stock instead of new Bf109 parts production (when it is needed, of course).