A pantyliner is an absorbent piece of material used for feminine hygiene. Wikipedia

A pad worn on the inner surface of women's underwear, up against the vulva, during a low-flow day of the menstrual period, designed to absorb small, spotty quantities of menstrual fluid, in contrast to a tampon or sanitary napkin, worn on heavy-flow days. Wiktionary

Pad worn inside underwear. Wiktionary (translation)