Aircargo
I will try to make this simple and easy to understand, without getting real technical. A PL or DPL tone is a sub audible tone that is basically "built in" to a frequency that an agency is using. Sub audible meaning we can't hear it on our scanners. When someone from that agency transmits on that frequency the tone assigned to that frequency is activated. Just about all frequencies have PL or DPL tones.
Many towns are assigned the same frequencies by the FCC. If you don't have the PL or DPL tone programmed into your scanner for a frequency for a town you want to listen to, you will hear all towns on that frequency that are within receiving range of your scanner. By programming the PL or DPL tone in, you will only hear the town you want to listen to.
I always use the example of Wall Township Fire and EMS, 154.400. New York City Citywide Fire is on the same frequency, and it is a strong signal. Witihout Wall's PL programmed in, I would constantly pick up NYC Citywide Fire, which is almost non stop and very annoying. But with the PL for Wall, 151.4, programmed in, all I hear is Wall.
If you read the manual for your scanner it tells you how to program these tones. In the manual it will say CTCSS and DCS tones. CTCSS is the same as PL, and DCS is the same as DPL. If you don't know what the PL or DPL tone you want for a frequency is, check the Radioreference Database. If you can't find the tone listed, your BC250D can search for the correct one. Again, it tells you in the manual how to do this. BTW, I have the BC785D, the base model of the 250D.
Hope you understand a little better. If you're confused or have any questions, just post and I or someone else will try to help you.