Will someone explain Bhavanga and Javana in simple way?
At times, they seem non-comprehensible.
According to the Ven. Bhikkhu Bodhi's book A Comprehensive manual of Abhidhamma, Chapter 3 Guide to §8 :
The word bhavanga means “factor (anga) of existence (bhava),” that is, “the indispensable condition of existence.” Bhavanga is the function of consciousness by which the continuity of the individual is preserved through the duration of any single existence, from conception to death. After the paṭisandhicitta has arisen and fallen away, it is then followed by the bhavangacitta, which is a resultant consciousness of the same type as the paṭisandhicitta but which performs a different function, namely, the function of preserving the continuity of individual existence. Bhavangacittas arise and pass away every moment during life whenever there is no active cognitive process taking place. This type of consciousness is most evident during deep dreamless sleep, but it also occurs momentarily during waking life countless times between occasions of active cognition.
In other words the Bhavanga-Citta is a mindstate that has all the necessary features of consciousness but is otherwise blank. It occurs in between all other mindstates and also during what we would now call being unconscious.
In the same section the Ven. Bhikkhu Bodhi explains Javana thus:
“Javana” is a technical term of Abhidhamma usage that is best left untranslated. The literal meaning of the word is “running swiftly.” As a function of consciousness, it applies to the stage of the cognitive process that immediately follows the determining stage 76 and consists of series of cittas (normally seven, all identical in kind) which “run swiftly” over the object in the act of apprehending it. The javana stage is the most important from an ethical standpoint, for it is at this point that wholesome or unwholesome cittas originate.
The Javana is one step in a series of things that occur after having a sense experience. A sense experience starts with the Bhavanga vibrating for one moment and in the next moment is cut off. Then a mindstate arises which turns towards the sense object; This turning is called āvajjana, or adverting. Then you have the appropriate sense consciousness arise that is aware of the sense object. If the sense object is one of the physical senses then you also have a mindstate of receiving, then a mindstate of investigating, and then a mindstate of determining that follow the sense consciousness. For a mental consciousness these three mindstates don't happen.
After this happens there arises a series of seven mindstates called the Javana. I think the best way to think of it is that in the earlier phases the mind is becoming aware of an object, and in the Javana phase the mind is actually reacting to the object. After the Javana you have some other phases, but they aren't that relavent to this discussion.
The bhavaṅga citta, mentioned earlier, is the primary form of mind. It flows from conception to death except when interrupted by a stimulus through one of the sense doors. When a stimulus enters, consciousness becomes active, launching into a thought process (citta vīthi). Thought processes have been analysed in great detail in the Abhidhamma. A complete thought process, occurring through the physical sense doors, is made up of seventeen thought moments (citta khaṇa). These are:
1) A bhavaṅga that flows by in a passive state when one of the five physical sense organs comes in contact with its object (atīta bhavaṅga).
2) A bhavaṅga that vibrates for one thought moment (bhavaṅga calana).
3) A bhavaṅga that cuts off the flow (bhavaṅga upaccheda).
4) A citta that turns towards the object through the sense door that has been stimulated (pañcadvāra-vajjana).
5) The appropriate sense consciousness; in the case of the eye, for example, eye consciousness (cakkhu viññāṇa).
6) Next a thought moment—the sampaṭicchana citta—which has the function of receiving the object.
7) When the object has been received another thought moment, called the santīraṇa citta, arises, performing the function of investigating the object.
8) The act (kamma) itself, especially if it was a weighty one.
9–15) The object having been determined, the most important stage from an ethical standpoint follows. This stage, called javana, consists of seven consecutive thought moments all having an identical nature. It is at this stage that good or evil is done, depending on whether the cittas have wholesome or unwholesome roots. Therefore, these javana thought moments have roots and also produce new kamma.
17) Following the seventh javana the registering stage occurs, composed of two thought moments called tadālambana. When the second registering citta has perished, the bhavaṅga follows, flowing on until interrupted by another thought process.
Source, Dr. N.K.G. Mendis, The Abhidhamma in Practice