To give a mental image, imagine that you have to fill a cup of water. There are two ways in which you can fill it at the sink: 1) open up the faucet so that a lot of water fills it up quickly or 2) let the water drip for a long time and fill up the glass that way. A camera works in a very similar way: that camera either lets in a lot of light or adjusts the time light is allowed to hit the film plane.
How the camera measures the time of an exposure is easy. A shutter opens up for a moment and closes. This is called shutter speed. Shutter speeds can vary from the very fast (e.g. 1/8000 second) to very long (e.g. 1 minute).
What is more difficult to understand is how the camera determines the amount of light into the camera. This is determined by f/stop.
In a lens there is a device called a diaphragm (normally a series of adjustable leafs that make a smaller or larger opening in the lens). When more light is needed, the diaphragm is opened. When less light is needed the diaphragm is closed. f/stops measure the size of the opening, which photographers call the aperture.
So why are they called f/stops and how do they work?