What Sun does in a chart
Sun is the part of the chart that answers one question:Who am I when I feel most like myself? It describes core identity — not events, and not outcomes.
What Aries does to it
Aries is a fire sign, so it moves on impulse and enthusiasm. It is also cardinal, which means it starts things. Put those together with Sun and you get identity through action — direct, quick to start, applied specifically to core identity.
Aries is ruled by Mars, so this placement tends to be coloured by drive and desire as well.
The useful version
Placements are not verdicts. This one describes a default setting, not a limit — the things listed above are what you do when you are not thinking about it, which is exactly why noticing them is worth something.