Easy enough to graph at wolframalpha.com:
This shows two signals: one at some frequency (blue), and another at 15 times that frequency (magenta). The brown line is the two added together.
If you took this brown signal and put it through a low-pass filter, you could recover the blue signal.
If you put it through a high-pass filter, you could recover the magenta signal.
Now, if you had many signals in here, separated by less than a factor of 15, it would be quite difficult to separate them by looking at the time domain signal. But this is why we don't think of this problem in the time domain: it's hard. Separating the signals in the frequency domain is easy.