The asteroid Juno was the third of the four major asteroids to be discovered, in 1805. In mythology Juno (or the Greek Hera) was wife and consort to Jupiter, and therefore the astrology of Juno represents the partnering energy of intimate relationship.
This is also the territory of Venus and the Moon, but where Juno seems to differ is in representing the partnership energy in a new light, as in the more modern issues of where the boundaries are with another person, how much sharing is appropriate, versus holding back, are there issues of bitterness and jealousy, or projected authority and control onto another person, and intimacy needs in general.
When Juno is prominent in a natal chart, all these type of issues are important to the individual. If Juno is aspected by another planet, these issues may be colored by the energy of that planet. Juno's position by house and sign is an indication of the area of life in which these issues will be most important.