1) what is a stake?
Staking is the process of locking your tokens for a fixed period of time. On mainnet, at the moment, this period is at least three days.
2) after making a stake, what cpu and ram gets used? it's said that after staking , you are using block producer's cpu and ram's resources. how am i using their resources at all?
When you stake, you specify how much of your stake is delegated for CPU and how much is delegated for NET. RAM needs to be bought separately on the internal ram market, and it does not need staked tokens.
You are using the resources of all nodes that process and store your transactions, as they compute and store the transactions on their own machines.
3) after making a stake, and transactions happen , my limit maybe goes down right? if i made a 1000eosio and it gave me 100 cpu, then let's say i finished it all 100 cpu resources. then what ? my 1000eosio is lost? so i lost the money? then how are transactions free?
You don't lose any of the EOS you stake; the bandwidth (CPU + NET) resources that you are entitled to decrease to stop you from spamming the network. However, within 3 days, the resources that you are entitled to replenish to their full value. You can unstake your EOS if you want and you'll still have the same EOS.
4) i believe after reaching my limit,i can still unstake and get the money back that i spend for staking 1000eosio => 5000$ . is that right?
Yes, you can unstake and sell the same amount of EOS, but there's no guarantee you'll sell them at the same exchange rate as the one you bought them for.
5) so I stake because to give network that i am willing to cooperate? or why do I stake?
Pasting my answer from a different question:
The economic purpose of staking is to provably commit to a promise that you won't sell the staked tokens for a pre-established period of time. This means that you undertake the duty of holding your tokens through the process of inflation as BPs mint more coins as a reward for their services. Therefore, staking is how you pay the producers for offering computational services: holding a digital token whose scarcity is perpetually affected by the BP reward system.
6) can I raise my stake after it's been staked already after 5 -6 days when reaching the limit?
Yes, you can stake more tokens at any point.