What is JSR and what's its use?

JSRs are Java Specification Requests, basically change requests for the Java language, libraries and other components.

It's all part of the Java Community Process, whereby interested parties can put forward their ideas for enhancements and (hopefully) have them taken up and acted upon. The process is detailed here.

For example, the Bluetooth one you mention is tracked here and the definitive list is maintained here.

Java Specification Requests (JSRs) are the actual descriptions of proposed and final specifications for the Java platform. At any one time there are numerous JSRs moving through the review and approval process.

Java Specification Requests (JSRs) is a formal document that describes proposed specifications and technologies for adding to the Java platform.