An oscilloscope (in short scope) is used to visualize signals (voltage differences) that are varying over time. One could see it as an advanced Digital Multi Meter, however it can only measure voltages, both AC and DC.
Why Do You Need One.
At first it might look a bit imposing, but once you have worked with an oscilloscope you will value this instrument. Because it will be your best friend when trying to debug your circuit. Giving you inside information from your circuit. Especially when working with micro-controllers (like Arduino) a scope is helpful; When you are debugging the software its often unclear whether something is wrong with the hardware connected to the micro-controller or with the embedded software. If you connect an oscilloscope to an output of your micro controller and run your program. You can now look if the outputs act according the program. If they don’t, keep debugging your software. If they do, a problem might be found in the hardware connected to it. You can start measuring there, and don’t need to focus on the software. In a similar way you can check the inputs. Keep in mind though that the software needs the right information from the inputs.
We have made a tutorial on using an oscilloscope.