In simple words
An aspect is an angle between two planets. A conjunction means they are about 0° apart.
It is a blending angle: the two planets merge and act as one.
In a birth chart
The planets involved strengthen and colour each other; the result depends on their natures. A conjunction is a focus point in a chart, often a talent or a theme that keeps returning.
Vesper counts a conjunction when two planets are within 8° of the exact 0° angle in a birth chart, and within a tighter range for day-to-day transits.
As a transit to your Sun
Sun conjunction your Sun. The Sun has returned to the exact place it held when you were born. This yearly return marks a fresh solar year.
Moon conjunction your Sun. The Moon is passing over the place your Sun held at birth. Once a month, your feelings line up with your core direction.
Mercury conjunction your Sun. Mercury, the planet of thought and speech, is passing your natal Sun. Your mind and your sense of self work together.
Venus conjunction your Sun. Venus, planet of love and beauty, is passing your natal Sun. Your warmth and attractiveness are heightened.
Mars conjunction your Sun. Mars, planet of energy and drive, is passing your natal Sun, boosting your will and your impatience together.