Does ART run apps code natively
Short story: yes, ART translates apps to code native to the actual phone's architecture and therefore apps execute natively without the need of a JIT compiler.

Long story: For reasons of compatibility, applications are still bundled in apks  carrying the java code compiled into dex files. But instead of just optimizing the .dex code and executing it inside Dalvik VM (DVM), the code is compiled down to native code. This is done on the device using a tool called dex2oat, where oat files are embedded within the regular ELF file format. 

The following figure depicts the difference between the usage of DVM and ART: 

 
(Wikipedia: Diagram of the Android Run Time architecture, recreated from information published at A Closer Look at Android RunTime (ART) in Android L.)

The performance improvement comes from the fact that the DVM used JIT compilation each and every time an app is executed, while AOT compilation only happens at install time.