Resonant Frequency is a concept from physics which describes a property of systems that cycle. For example pushing a child on a swing (simple pendulum).
Each swing has a characteristic resonant or natural frequency. When energy (in the form of a push) is added at the resonant frequency, there will be a large change in the amplitude of the cycle i.e. the swing will go much higher. If the push is a little slower, or a little faster than the resonant frequency the swing will not go as high.
In the context of Heart Rate Variability feedback, the resonant frequency is the rate of breathing that results in the largest swing in heart rate. In humans the resonant frequency depends on the individual, and usually lies between four and seven breaths per minute.
It is possible to approximate the resonant frequency in a person simply by having him breathe for a period of time at a given rate, say 5 breaths per minute, then increasing the rate to 5.5 breathes per minute etc. The breathing rate that creates the greatest variation or "swing" in the heart rate is the resonant frequency for that person.