Lucky you!
If you're new to the idea of PWM then maybe it's not obvious.
PWM = pulse width modulation.
Basically, adjusting the time "on" and "off" of a series of pulses, to change the average.
The solar charge controller monitors battery charge and regulates the current from the solar cells if there's too much. As long as the batteries are accepting full PV current without going over-voltage then the charge controller doesn't have to do much. When the voltage does come up to a specified level (say, abut 14.0V for a sealed lead-acid battery), then the charge controller will reduce the current flowing to the batteries. It does this by shutting off the PV current for a fraction of a second, then back on again, off, on, off and so on. If it pulses on half the time and off half the time, then the average current that flows is 1/2 of what it would be if the PV was hooked up straight to the battery. This is good, because by regulating down to 1/2 the current, the PV will not over-charge the battery, now that it's mostly charged, and the process of bulk charging or float charging can go on under control. So, on a day with full sun, straight on the panels, the current could be 100% if the charge controller detects that the battery is discharged, or 5% if it detects that the battery is full.
How often pulses on or off is what's determined by the frequency switch. Your CC can pulse at 25/50/100 Hz; select whatever suits you; if you hear static on the kitchen radio, try a different setting. The charge regulation will happen no matter what frequency is set. On a Tristar, the PWM can be shut off entirely, but that's not recommended. They suggest you try other noise suppression devices before hampering the operation of the charge controller.
Were your benefactors able to provide you with a user's / installation manual? I found it extremely helpful in tuning it just right (and avoiding mistakes) when setting up the Tristar I got a few years ago.
