Meaning of negative frequencies in Baseband non sinusoidal, non periodic signal I can understand the meaning of negative frequencies in a sine or cosine signal, since by using Euler's identities, you have two complex phasors moving to different directions, which when added, give a real (cosine or sine) signal.
However if I have a random baseband (real, not periodic) signal, what does it mean for it to have negative frequencies?
It means the same thing.
The negative frequencies are there to cancel out the complex portion of the signal in the time domain, so that the time domain signal is constrained to the real axis.
The Fourier Transform decomposes a signal, $x(t)$ onto a set of complex basis functions: $e^{-j\omega t}$ for $\omega \in (-\infty, \infty)$.
If $x(t)$ is real, then the decomposition, i.e. the Fourier Transform $X(\omega)$, must be such that the complex portions cancel out, when the inverse transform is performed to recompose the signal from it's constituent parts.
This is a conceptual explanation of why the Fourier Transform of a real signal is symmetric about $0$ Hz.