The phrase bolt on comes from the mechanical world where a machine has a connection that can be used for multiple purposes simply by bolting on or attaching the correct additional part. Bolt On in software typically refers to additional software that is not part of JDE, but the vendor understands JDE and the software has built in connections and/or documentation that make it very easy to setup and use it with JDE. This is usually accomplished by installing an ASU the vendor provides or creating database connections as detailed by the vendor. This is often an advantage over software that will work with JDE but you have to figure out and create all the necessary pieces to make JDE and the other software work well together because it saves time during installation.
You may hear some people refer to additional JDE software as bolt on, even though it is created by Oracle specifically for JDE but not included in base licenses, BSSV and AIS would be examples.