People like it when you understand them completely. Lots of nodding and "yeahs" are required for any conversation. For fun you can throw in "I got you" and "Sure thing". Never under any circumstance can you say that you don't understand. Even if you don't follow the person. In some people this can create real anger. 'Are you trying to say that I am stupid with your misunderstanding', 'Why don't you already know what I have to say', 'Why should I catch you up on what I am saying -- why don't you have the same life experience as me?'
I often have no clue what people are taking about and when you ask them to explain further, they get really edging and defensive. Like you are saying "Why would anyone say that, you are ridiculous". But just maybe I don't know where nuclear power comes from, or the hierarchy structure of a given company (including my own). Perhaps not everyone knows not to wear white after labor day or that children should be reading by age 2. Not that I disagree, but just perhaps I am totally clueless.