An integer PLL has integer dividers. If you have a 10 MHz reference crystal, and a 1000 MHz VCO, as an example:
R=10 reference divider
N=1000 VCO divider
So the reference becomes 1 MHz, and the VCO after dividing down is also 1 MHz. Being the same frequency, the PLL can lock the two up, and make the 1000 MHz oscillator be very stable and accurate.
If you program N to change to N=1001, now the VCO moves from 1000 MHz to 1001 MHz.
A fractional PLL can use non integer divisor ratios. If N=1000, you can program it to be N=1000 1/32, 1000 2/32 for example, and get a much smaller step size of 1/32 MHz.