This is a FM MODULATOR or a VOLTAGE TO FREQUENCY CONVERTER.
The Control Input of the 555 is fed with a square/sine/sawtooth voltage to modify the internal trigger-threshold of the RC oscillator, changing the time between output pulses, therefore modulating the output pulse frequency. The R and the C can also be adjusted to set the desired starting maximum frequency. Note that in each case, the Output Frequency is directly proportional the Amplitude of the Input Voltage (the modulating signal). In other words this is an Pulse Frequency Modulator.
