The point is reverence, not display. Clothing should help you pray, avoid unnecessary distraction, and show respect for the altar, the Liturgy, and the faithful around you.
What modesty means here
Modesty means simplicity and respect before God. It directs attention away from style, expense, or cultural taste and toward prayer. Different parishes may have different local expectations, and climate or practical circumstances may also matter.
For most people, simple pants or a skirt with a modest shirt works well. Bring a light layer if you are unsure. Comfort matters too, because you may stand for long portions of the service.
How to handle uncertainty
If you arrive and realize others are dressed more formally, do not leave in embarrassment. Come in, pray, and learn. If you plan to return regularly, ask someone trustworthy what is customary in that parish.
Over time, the question becomes less about minimum acceptability and more about preparing the body for prayer. The Church teaches through the body: standing, bowing, fasting, covering, silence, and modest dress all help the faithful remember that worship is holy.
Choose clothing that supports attention before God and lets the service, not the outfit, become the focus.