Both Ardor and Ignis work together as the solution for "NXT 2.0"  Ardor will be the main chain and Ignis will be the 1st child chain.  Ignis will port nearly 100% of the state of NXT over to its genesis via a snapshotting process.

Fees on Ignis will be VERY low.  Fees on Ardor will be pretty high.  This allows the system to prune heavily on Ignis, solving the blockchain bloat issue that has plagued every other cryptocurrency out there.  The main chain Ardor will secure the rest of the network (all child chains).