Definitely a long question...will try to answer one at a time:
-What is a conjunction?
-How does a conjunction between planets affect the native y who has this placing?
"The conjunction is formed when two planets are at approximately the same degree of the same sign or at an orb of not more than 8 degrees. In a conjunction, the energies of the two planets are combined, and the result is often a strengthening of both....A Moon-Venus conjunction is beneficial whereas a Saturn- Mars conjunction is said to be stressful." (1)
* So, this is indeed a "stressful" meeting, as Uranus gives unexpected change, revolution, and the vision of the populace and is applying pressure to Neptune, the deep, dark, hidden things....this is considered a "transformational aspect" for you, in that, activating it will literally change your entire life..*
-What does Uranus Conjunct Neptune mean?
*This aspect will gift you with a wonderful imagination, flashes of insight, and sometimes, genius or prodigies. (2) Psychic ability can be pronounced here. *
-Is a planets conjunction good or bad?
*No, but it can feel like it sometimes. Those "stressful" conjunctions are meant to spur emotional growth, and sometimes that can be painful. *
-Does a conjunction make someone more powerfull?
*Rarely. Usually, a conjunction provides an avenue of development for that person, a tool that person can use or a challenge for that person to overcome. In your case, it's probably a tool that will manifest if you "activate" it by developing it; actively accessing it...like a muscle you work out to keep it strong and make it stronger...in this case, this aspect will probably manifest as a PERSONALLY powerful one for you, but not so much for other people...*
-Uranus Capricorn 22°04'56 in house 8 direct
*This placement usually gives the person a serious businesslike outlook...in rare cases, may actually bring out that rare, wild, insanely ambitious side of Capricorn. (think cut-throat 80s...)..definitely not a comfortable place for Cappy to be...and this will be seen in your sexual relations, your mate's business relations and in esoteric practices....*
Sun in Aries actually amplifies the wildness of the Uranus conjunct, as its a "think first, ask questions later" type of position..and the Moon in Sagittarius stimulates the overconfidence and overly optimistic outlook...which can be disasterous when applied to the aspect...
So, no business speculations for you! No high stakes gambling..and definitely no no no nasty, cutthroat business deals....bad mojo..
Whew....I'm tired now! See below for further reading on your specific placements!
Blessings to you...