In this case, it's a way for the chip to generate an on-chip clock signal that's a multiple of the external clock signal. (PLL = phase locked loop; it's a way to lock one oscillator to another. They're used all over the place, especially in radio.)
Go to http://www.nxp.com, in the index choose microcontrollers>16-/32-bit microcontrollers (ARM7 core)>your chip (example: LPC2138)>Features and download the application notes. There's one appnote that explains how to use pll.