What are Bhavanga and Javana? 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.