Sorry - not an expert, nor am I terribly familiar with all the options in the editor. My best shot at explaining the two items is:
(1) The chance for a city's citizen to resist (become or continue to be a resistor) when the city is conquered is influenced by a number of factors -- one such factor is total culture, another such factor is the government chosen by both the citizen's previous ruler and the conquering nation. The "resistance modifier" is an editor variable that adjusts the chances of resistance based on government type. By increasing default values the modder would would increase the chances of resistance -- for example, if the conquering civ is a democracy and the conquered city was previously ruled by a Monarchy, the default resistance modifier of -5 means that, all else being equal, an individual citizen would be less likely to remain a resistor. If you instead adjusted the number from -5 to +25, the chance of continued resistance from each citizen would be higher. I suspect, but do not know or claim to know that the rsistance modifier works on a straight percentage basis with the "resistance chance" value in the "culture" tab of the editor -- i.e., if the continuing resistance chance was 10% and the resistance modifier was +5%, I suspect that any given resistor would have a 15% chance (10+5) factor applied to whatever the base chance of resistance was.
(2) Propaganda modifier is, I suspect, similar. It adjusts the base chance of propaganda success depending on types of competing governements -- with a democracy, the propoaganda modifier against a communist nation is 10, which I assume is 10%; versus a Republic it is only 5. I presume that this means that propaganda attempts by a democracy have a better chance of success against a communist country (bonus 10) than a republic (bonus 5). Exactly how the chances of success are computed, and how sensitive the editor-configurable values are, I do not know.
Again, only my guess-timates -- experienced modders can probably offer more detail and correct my inaccuracies.