An emitter is an object that was introduced in Peggle Nights. They cannot be used in the first Peggle game. Emitter’s create visual particles which add dynamic effects to the level. They do not change the gameplay in any way and generally just make the level look nicer.
Some examples of where emitters are used are:
Emitters often have movement info associated with them.
There are a number of properties that can be used to change the behaviour of the emitters and their particles.