What role if any do javana play in learning and entering the jhanas? I'm reading a book on jhana and javana was brought up but i couldn't really understand the role it play in the jhanas.
I would say that this word has been conflated and in my experience unnecessary for entering jhana. please refer to the instructions on how to enter jhana found in the Sutta Pitaka.
This is picture that show consciousness's arising one by one (consciousness can arise just one per time per person.)
Javana is Pr to Jh in the below picture.
When bhavaṅga vanish, then manodvāra arise. When manodvāra vanish, then parikamma javana arise. When Pr javana vanish, then Up javana arise. When Up javana vanish, then An javana arise. When An javana vanish, then Go javana arise. When Go javana vanish, then Jh javana arise. When Jh javana vanish, Next Jh javana arise. Infinite Jhāna javana arise until jhāna will stop.
Parikamma is practicing consciousness. Upacāra is up level of parikamma. If Upacāra can up level enough to change to be appanā-jhāna, then gotrabhū will arise to prepare for jhāna. The arising step of consciousness will be as alike as the below picture.
But if Upacāra can not upgrade to jhāna level, 1 parikamma javana arise then 6 Upacāra javana arise, step by step. Then bhavaṅga arise, infinity. Then loop again. In pali canon, teachers just called them the same as "javana", because no arising of jhāna-consciousness, equal to nothing.
This loop will be Upacāra-samadhi when the practitioner has not any unwholesome loops arise between the wholesome loops. If just one unwholesome loop arise, the practitioner still being at khanika-samādhi level.