An ID Key allows to store custom values on a per object basis. The Id Keys system got a major update.
Until now there was only one ID Key by default (Initial Transforms). Now there is a second one (Index) which can be used for all kinds of things. E.g. it can be used to sort objects before passing them to Animation Nodes. There are a couple of operators that help with that.
It is possible to create a new ID Key now. Note that it might disappear after a restart when it is not used anywhere. Furthermore some ID Keys can be removed as well. Wether a ID Key can be removed depends on where it is used. It cannot be removed when a Object ID Key node uses it or when it is one of the default id keys.
Animation Nodes currently supports 4 different types of ID Keys: Transforms, Text, Float and Integer. All of these can be accessed using the Object Id Key node.