There are five major aspects in astrology: the conjunction, opposition, trine, sextile, and square. This post will specifically talk about the conjunction.
A conjunction (☌) is when two planets in a chart is 0 degrees of each other. This would mean that two planets will be in the same sign and house, unless you use certain house systems like Placidus where there are exceptions. Sometimes, with various house systems, you can get off-sign (or dissociate) aspects due to creating aspects out of location rather than the signs. In-sign aspects will be of focus.
The conjunction is considered the most powerful aspect in astrology. Due to its power, lots of astrologers assign a larger orb to conjunctions. The conjunction (by itself) is also the only aspect that can be seen as “neutral” (as opposed to harmonious or disharmonious.)
With the involved planets being in the same signs, this means that the two planets will have the same element, modality, and polarity. What you essentially have is two facets of your self-expression having the same goals. The power of two planets becomes a singular unit. This is where the power of conjunctions come from. Other than the unique characteristics of the planets, there’s little to no differences.
Disclaimer: Even though a conjunction can strongly bind two planets together, it does not mean the planets lose their sense of autonomy to the aspect. The planet is independent, but makes numerous relations to other planets in the chart.
Now, the conjunction by itself is neutral, but this neutrality will inevitably change once you look to the actual planets involved and the signs they are in. Looking at the essential dignities can definitely show whether the conjunction will be comfortable or uncomfortable.
As for the actual manifestation of the conjunction, whether it will be positive or not depends on the planets involved and the individual. It could go either powerfully well or not.
Here is a visual example of a conjunction. Mars is in 6 degrees Aquarius while the Moon is in 13 degrees Aquarius. This would be Moon-Mars conjunction with an orb of 7 degrees!