What does the expression 'combine signals coherently' imply?
It means to combine signals taking phase into account (with respect to some mutually related reference point). Taking phase into account allows signals to sum when in phase, or cancel when out of phase (with respect to that related reference point).
For instance, when summing the result vectors of successive FFT frames together, you can just add the magnitudes, which would be an incoherent combination, or sum the complex vectors, which would give you a coherent sum. With the coherent combination, any stationary sinusoids with a frequency exactly between 2 FFT bins across 2 successive FFT frames would cancel out instead of summing its spectrum, thus better rejecting that spectrum if it is undesired.
In the time domain, it might mean adjusting the phase of a PLL to match that of an estimate of some carrier's phase, so that a signal combination maintains some coherent relative phase relationship, instead of the relative phase being yet another unknown into some modulation, demodulation or detection process.