I was talking to my daughter about that yesterday. Suppose Facebook knows your interests, taste in music, where you live, what good you like, etc. A political campaign could use that information to tell you that their local candidate shares your interests, lived in the same town, loves the same music, supports the same charity or whatever overlaps with your profile, while knowingly avoiding telling you about things you like or are in favour of for which they hold opposing views. So you get a personally tailored, custom ad for the candidate pushing all the right buttons and concealing anything that doesn’t match or it knows you would dislike. Meanwhile it could also show you targeted attack ads on a rival customised to highlight things they know you dislike.
All the information might be true (or might not), but IMHO I don’t like the idea of people intrusively trying to manipulate me like that. We all have biases and preconceptions. We’re all open to manipulation and the last thing I want is my online world to become an echo chamber, turning me into a parody of myself. In the wider context, it’s also a threat to civil society, driving a wedge between us as citizens by magnifying our differences and promoting divisiveness. That’s what the Russian interference campaign was all about.