The light sensitive resistor or photocell as I like to call it is just that. The resistance changes depending on how much light falls on it. It could be used to switch a light on when it got dark etc.

It is used in conjuction with another resistor of say around 10kΩ and the resistance is split between the two of them. Similar to the variable resistor (potential divider). The current passes through both at the same time and what you measure is the voltage drop or gain across one of them.

If you are struggling to understand what I mean then I suggest you google it and there are plenty of examples to show you how to nurse it with the Arduino Uno. It would take a little too much time to explain here in detail.