In English-language countries, positive ads (advertisements) and negative ads describe ads that are either a) ads that are positive about the candidate the advertisement is meant to support, or b) ads that are negative about the candidate's opponent, tearing the opponent down with insults and attacks.
Example: In American politics, candidates who are safely ahead usually air positive ads that advertise their own achievements and virtues. Candidates who are threatened have, in recent years, aired large amounts of negative ads attacking their opponents as morally, intellectually, and politically flawed persons who are not deserving of being elected.
Negative ads have raised the general level of cynicism about politics. This is the context in which they are described in the Western media.