Const is a way to mark data as constant or in other words read only. This way, if you try for example to write to a variable, the compiler warns you about it. So essintualy the compiler helps you find bugs. But it is more than that. It gives you confidence, that, when you pass pointers to a function (more on that later), the data wont be changed afterwards.

Your task is to first declare the variable pi as constant, since pi should never change and execute. Then read the error message and correct the mistake. This may seem silly with so few lines of code, but when you have 1000 of lines of code, it get harder to find errors like this.