I looked in Ethereum scan and saw often a red marked with transactions telling "Out of Gas" What happens with such transactions?
The gas is the number of basic operations a contract has to execute. When you want to execute a contract, you give monney for the gas and you fix the maximal price of the gas.
When a miner accepts to take your transaction, he executes the operations until either it terminates or it consumes all the gas. In this second case, the transaction is out of gas. You loose all the gas money you sent and the transaction is reverted.