For the most part, they are obstructing terrorist fighters trying to enter Syria, and blocking terrorist arm shipments.
USA troops are also defending the relatively small Syrian oil fields north and east of the Euphrates River.
What is Europe doing to help out?
Nothing that I know about.
The US-led coalition has been operating against ISIS in both Syria and Iraq since 2014. It includes several NATO allies: Belgium, Canada, France, Germany, The Netherlands and the UK. NATO has not contributed to airstrikes as an organisation but they have been carried out by US, the UK and France.