JCR stands for Java Content Repository. It's an API produced by the JSR-170 comitee.
The main (and reference) implementation is Apache JackRabbit.
The structure of OpenWFE data is hierarchical, it's natural and easy to accomodate it within a JCR.
The JCR is a Java API and OpenWFE is written in Java.
OpenWFE is used within the Magnolia CMS/DMS which stores all its content in a JCR, another link.