Resistors are the brakes of electronics. They slow down the flow of electrons (the current) in much the same way as the faucet slows down the flow of water when you slowly close it. Resistance is measured in Ohm because a bloke called Georg Simon Ohm said so. The resistance is a measure of how difficult it is for electrons to push through the resistor, or in our water-and-pipes analogy, how open the faucet is. The higher the resistance, the slower the current will flow.
The values of resistors are written on them using colour codes like this:
here is another calculator.