To complement the previous post, I will comment on what firing rate means in spike-based theories. First of all, rate is important in spike-based theories. The timing of a spike can only exist if there is a spike. Therefore, the firing rate determines the rate of information in spike-based theories, but it does not determine the content of information.
A related point is energy consumption. The energy consumption of a cell is essentially proportional to the number of spikes it produces (taking into account the cost of synaptic transmission to target neurons) (Attwell and Laughlin, 2001). It seems reasonable to think that the organism tries to avoid any waste of energy, therefore a cell that fires at high rate must be doing something important. In terms of information, it is likely that the amount of information transmitted by a neuron is roughly proportional, or at least correlates with its firing rate.
For these two observations, it follows that, in spike-based theories, firing rate is a necessary correlate of information processing in a neuron. This stands in contrast with rate-based theories, in which rate is the basis of information processing. But both types of theories predict that firing rates correlate with various aspects of stimuli – and therefore that there is information about stimuli in firing rates, at least for an external observer.