In spoken German native speakers frequently make use of modal particles. Modal particles express what the speaker thinks of what he is talking about. They are words like ja, doch, wohl, aber auch, schon. It is difficult to understand these words as there is not one translation for them but many. The meaning of a modal particle changes with the context and sentence (you could say they are untranslatable).
If modal particles change their meaning, how to understand them?
To understand a modal particle you need to (1) think who is the speaker of the sentence (2) what common knowledge both of you have. As modal particles are used to express the attitude of the speaker, if you understand the context of the sentence you can often guess the meaning.
When you have no idea about the meaning of a model particle, just ignore it first. The sentence will make sense without it. Then think about what the speaker probably thinks of what he is talking about. Maybe that’s what he/she is trying to express with it.
I find that Wikipedia does a great job in explaining modal particles. Here are the two most important articles
General Information about Modal particles –
Specific information about Modal particles in German –