What it is
The Sun sign is the zodiac sign the Sun was passing through on your birthday. Because the Sun spends roughly a month in each of the twelve signs, your birth date alone is usually enough to know it. It is the only part of a birth chart that horoscope columns use, which is why it became the sign people identify with.
The dates shift by a day or so from year to year, so if you were born near the boundary between two signs — the cusp — the exact time and place matter. A chart calculated for your actual moment of birth settles it; the Sun is always in one sign or the other, never both.
In traditional astrology the Sun stands for vitality, purpose and the way you want to be seen: the core self you return to.
Why it matters for daily reflection
The Sun sign is a broad brush. It will not explain a Tuesday afternoon. But as a starting image it is useful: a Virgo Sun who notices that she calms down by tidying, a Sagittarius Sun who realises he gets restless when a week has no open road in it. The sign gives you a hypothesis to test against your own days.
It also helps to hold it loosely. Your Sun is one voice in a chart that also has a Moon, a Rising sign and eight planets. If a Sun-sign description never fits, that is information too — look at the rest of the chart first.
In Lunara
Lunara's basic natal chart, included in the free tier, shows your Sun sign alongside the Moon and planets. It is one of the fixed points that today's Moon rhythm is read against when your daily Signal is composed. The Sun sign is available even when your birth time is unknown, since it depends on the date.