Can anyone explain ?

[Jane]

Datasheet for a CPU says about a pin

VCXO PLL Power Supply pin
and about the other
 PLL Power Supply pin

Can anyone explain what  VCXO and PLL are used for?

Thank you

[Jimmy]

VCXO = Voltage Controlled Crystal Oscillator

[TerminalJack505]

VCXO = Voltage Controlled Crystal Oscillator

And a PLL is a Phase Locked Loop.  The CPU likely has a PLL so that it can internally generate a frequency that is a multiple of the external crystal.  An 80MHz clock can be generated internally from a 10MHz crystal, for example.

[Kremmen]

As the guys say. One point of a PLL is that you can generate significant frequencies from a much lower one; for instance just now i have a Spartan-6 FPGA on a breadboard under development. From a 12 MHz crystal it can generate internal clocks well beyond 100 MHz using a PLL and programmable dividers. After a certain limit i guess crystals will become impractical as baseband frequency sources, so you will want to use some kind of multiplication scheme. A PLL is one and works very well.

[TerraHertz]

Above are correct. But another important point is that both VXCO and PLLs are extremely sensitive to power supply noise and variations. Hence these pins are brought out separately and MUST be provided with well filtered clean and very stable voltages. You may find that the grounds for these circuits are also brought out separately, and the same cautions apply.
Typically you'd have a separate small IC linear regulator just for these rails, with R-C or L-C low pass isolation from everything else on the board. And they'd better be as close to the chip as possible, AND use their own RF-isolated ground plane segment.