Can you be more specific as to what you don't understand? It's talking about how a Fourier Series exists for any function over a given interval. For example consider a square wave function - you can find the series that is equivalent to a repeating square wave from t=0 to infinity, including the interval from t=0 to 10 seconds. Now imagine a second function consisting of a square wave from t=0 to t=10, then at t=10 seconds it changes to a sawtooth - you can find the Fourier Series that runs from t=0 to t=20 consisting of the square wave and then the sawtooth wave, and it's a different series than the first one, as that was a pure square wave. So now we have two different series that both result in a square wave for the first ten seconds. With this reasoning you can see that there are an infinite number of functions that have a square wave shape from t=0 to t=10 but have some other shape after t=10. Hence there are an infinite number of Fourier Series that consist of a square wave from t = 0 to t=10. Does this help?