The PropellerAds.com threat is adware or browser redirect virus. It may slow your computer down siginficantly and display advertisements. The main idea is for your information to likely get stolen or more ads to appear on your device.
The creators of such unwanted apps work with pay-per-click schemes to get your computer to visit risky or different types of websites that may generate them funds. This is why they do not even care what types of websites show up on the ads. This makes their unwanted software indirectly risky for your OS.