"Isotope shift" refers to the shift of the resonance frequency of the fluorine nucleus, depending on whether it is bonded to 12C or 13C. The different carbon isotopes to which the fluorine can be bonded cause a difference in chemical shift.
As mentioned by Kirk Marat, these shift shifts can, in the case of measuring fluorine, be quite large. In routine cases they are often very small, so the difference between the frequency of proton signals bonded to 12C or 13C is negligible, so that the appearing doublet for the 13C case has its central frequency at the same point as the singlet for the 12C case. This results in symmetric satellites. For 13C-19F this difference becomes significant, meaning that the central frequency of the 13C-19F doublet does no longer coincide with the 12C-19F singlet frequency. This can then be seen as asymmetric satellite peaks in the spectrum.
