Definition of bolt-ons:
(n) :
(figuratively) A functionality or service that can be added to an existing arrangement or plan (for example, a mobile telephone subscription).
(n) :
(also figuratively) An accessory that can be bolted on or otherwise attached.
(adj) :
(figuratively, computing) Used to extend an existing system, operating as an add-on or plug-in.
(adj) :
Of an accessory: capable of being bolted on or otherwise attached.
(n) :
(slang, mostly plural) A breast implant.