Inverse Kinematic , in the case of 3d animation, is a technology that permit the engine to estimate roughly the positions of the different part of a body between joint, where only the joint are controlled.
In resonite its what make you arm move in vr, it use your head and hand positional data to estimate the rest of the arm position (like elbow).
Technology
Currently resonite use a port of FinalIK heavely patched for the FrooxEngine.
Future
This is to be changed, it was scheduled to be the next big rework in resonite for 2024. But following the May 2024 survey which was asking if next priority would be IK or performance, performance won, that mean we don't know when will IK rework be put on the table again.