If you spend enough time looking at the sample code in the Unity Scripting Reference, you'll notice that a vast majority of the code is executed using the Update() method. As your game runs, the Scene window is displayed many times per second, which is called the frame rate or frames per second (FPS). After each frame is displayed, the Update() method is called by Unity, making it one of the most executed methods in your game. This makes it ideal for detecting mouse and keyboard input or running gameplay logic.
If you're curious about the FPS rating on your machine, hit Play in Unity and click the Stats tab in the upper-right corner of the Game view:
You'll be using the Start() and Update() methods in the lion's share of your beginning C# scripts, so get acquainted with them. That being said, you've reached the end of this chapter with a pocketful of the most fundamental building blocks programming has to offer...