Let's 1st understand what these planets represent on their own?

Mercury - Mercury represents communication skills, marketing, calculative ability, younger siblings, hobbies, skills with hands, intelligence, quick decision making, logical thinking, youthfulness etc.

Jupiter - It is the most benefic planet and it represents all the auspicious things like Knowledge, Wisdom, Law, Guru, Spirituality, Religion, Philosophy, Literature, Elderly People and Children etc. For a girl, Jupiter also represents Husband.

House Ruler - Besides these karaka things, Mercury & Jupiter will also rule some houses in your chart and will represent things related to that house. Of course we can't cover conjunctions Ascendant and house wise else it will never be over. So, you have to include those things at the time of interpreting this conjunction. Also, we need to keep in mind all Yogas and Raj-Yogas which their conjunction would be making to completely understand the conjunction. Here, we can only cover meaning as per karakas. 

Meaning of Mercury-Jupiter Conjunction - Mercury & Jupiter both represent Teaching & Counselling. So, 1st of all this conjunction shows someone who is very learned, knowledgeable and in academic field. This shows a person who is forever student and always likes to collect information. His knowledge base can be super high. He will work like Teacher, Professor, Preacher or Guru. He will travel to different places to get his knowledge. He can have a very wise or spiritual way of communication. They can be excellent counselors on any life matter. So overall, it is a knowledge-based conjunction and knowledge will be regarding the house/sign this conjunction falls in. Like in 2nd house of wealth or in Venus ruled sign, person can have excellent knowledge of Finance, Wealth and Investment but in 9th house/12th house or Jupiter ruled signs, it can be knowledge regarding Religion and Spirituality. In 8th house/Scorpio, it can be knowledge related with Occult. At the same time, Jupiter-Mercury are mutual enemies and hence, this conjunction shows a kind of frustration in individual as he feels that he didn't get proper reward of his knowledge. Also, as Jupiter was foster parent of Mercury, these people may have an inclination towards adopting a child. 

Who will win the Conjunction? - Jupiter will. Mercury is a neutral planet. It changes itself as per the planet it sits with. So, with Jupiter, Mercury's communications will be blessed with Wisdom and Knowledge. 

Dignity & Strength - 

This conjunction works best in Leo as Jupiter & Mercury both will be in friendly sign. 

In all other signs, either Mercury will be in bad dignity or Jupiter will be in bad dignity. Still as Jupiter is most benefic planet and Mercury is converting itself in Jupiter, this conjunction will be beneficial in result in most cases. In every sign/house, they both will start gathering knowledge related to that sign/house and then spreading that knowledge. 

Pisces is another sign where this conjunction is good as I never bothered myself for Mercury being debilitated. This person can gain a lot in Spiritual Domain of life.