Modal particles are words that don't have just one clear-cut and specific dictionary definition. But they play a very important part in the meaning of a sentence. They're predominantly used in spoken language (discourse particles). They convey additional meaning and signalize the speaker's point of view of what is being said. Some of the most common modal particles are "doch", "denn" and "eigentlich".

Modal particles don't refer to a particular part of the sentence, but instead to the sentence as a whole. Their meaning depends very much on the context.

Examples:
Kommt doch rein, Kinder. (freundliche Aufforderung)
Wo ist denn Julia? (Erstaunen)
Wo waren Sie denn schon? (Interesse)
Ich habe eigentlich erst Ende der Woche mit euch gerechnet. (Erstaunen)
Nach Dresden müssen die beiden eigentlich auch noch fahren. (Idee)