In simple words
An aspect is an angle between two planets. A opposition means they are about 180° apart.
It is a challenging angle: the two parts of you pull against each other, which builds strength when you work with it.
In a birth chart
Oppositions are often felt through relationships, as if another person plays one side. The work is balance: holding both needs instead of swinging between them.
Vesper counts a opposition when two planets are within 8° of the exact 180° angle in a birth chart, and within a tighter range for day-to-day transits.
As a transit to your Sun
Sun opposition your Sun. The Sun is opposite your natal Sun, six months from your birthday. Others become a mirror for your own purpose.
Moon opposition your Sun. The Moon is opposite your natal Sun, like a small personal full moon. What you need and what others need face each other.
Mercury opposition your Sun. Mercury is opposite your natal Sun. Other people's ideas and voices face yours directly.
Venus opposition your Sun. Venus is opposite your natal Sun, so relationships and other people's wishes stand across from your own.
Mars opposition your Sun. Mars is opposite your natal Sun, so other people's drive confronts yours.