From your comment
why the shape in the frequency domain looks like a bell shape.
Like @MarcusMuller said: because it looks like that
Yes, if you want, you can say that's because multiple different frequencies combine together. Since $X(f)$ is continuous, it's actually an infinite number of frequencies with different contributions that combine together to give you this specific shape. Other signals have frequencies that combine differently and give you different shapes in the frequency domain like triangle, square etc
As a side note, if whoever made the plot intended to show the magnitude response $$\lvert X(f) \rvert$$ as opposed to the frequency response $$X(f)$$ Since $\lvert X(f) \rvert$ is symmetric, that would imply a real signal $x(t)$.
However, if indeed the picture intended to show the frequency response $X(f)$, then since $X(f)$ is even (but could be real or imaginary), that implies that $x(t)$ is either real and even ($X(f)$ real and even), or imaginary and even ($X(f)$ imaginary and even).