PLL locks the free-running oscillator to a certain frequency using the VCO's tuning voltage pin by adjusting the voltage of that pin. PLL controls the output voltage of the charge pump so that VCO stays "locked" at that frequency and phase. And this is done in a certain bandwidth, how much of a free bandwidth that you want to give the VCO to freely roam around. If you put a small value on Loop BW, "lock" will take much time, and if you put a large value on Loop BW, then it may lock faster, albeit at a increased cost on "preciseness" of your output frequency.