After receiving the temple endowment, adult Latter-day Saints begin wearing a special undergarment beneath their regular clothing. It is commonly called the temple garment.
Latter-day Saints view the garment as sacred. It serves as a daily reminder of the covenants they made in the temple and of their commitment to follow Jesus. LDS teaching also connects it with spiritual protection.
Because the garment is deeply personal, Christians should not ridicule it or ask intrusive questions about how someone wears it. If an LDS friend brings it up, listen respectfully and allow that person to explain what it means.
The more important witnessing conversation is not about the clothing itself, but about the covenants it represents and where we find confidence that we are accepted by God.