what does frequencies in non periodic signals mean?
The physical siginificance of the Fourier transform for nonperiodic (finite energy) signals is not something clear from any point of view.
The limiting case of a Fourier transform of a periodic signal, as the period goes to infinity, consists of line components (dirac impulses at a countable set of discrete frequencies) which becomes more and more crowded, eventually approaching a continuum of frequencies, at which are no more dirac impulses.
This infinitesimal frequency component at any $\omega$, has no physical significance, unlike the dirac impulse at $\omega$, which has a direct implication of a sinusoidal signal component, extending from minus infinity to infinity (in time for example).
Non-periodic signals are also known as transients, or non-stationary signals, and for their analysis, Wavelet transform family can provide a better insight than the Fourier family.
Nevertheless, the Fourier family of transforms can still be used to analyse periodic as well as nonperiodic signals with great success.