Basically, the EU Commission realized that air quality - especially in cities - was getting worse and worse. The main reason for that being pollutant emissions especially from diesel cars. The evil emission the Commission is (among others) trying to reduce is called nitrogen dioxide (NO2).
Well - because you are breathing the air in your city! And more than 5.5 million people worldwide are dying every year because of air pollution.
Currently the European Parliament and the Council are negotiating in trialogue and an agreement had to be reached already in spring 2016. Even though this dossier is highly technical - and nobody really cared about it - it became quite famous during the past months with the VW scandal happening and air quality being a "hot topic".