A serum is usually a water-based, lightweight product concentrated in certain ingredients. Its fluid texture spreads easily and adds targeted support to a routine.
It's typically applied after cleansing and before moisturiser — following the thin-to-thick rule. A few drops usually suffice; let it settle briefly, then apply moisturiser.
Serums aren't mandatory; how comprehensive a routine is depends on skin needs. "Less but right" applies — layering many serums can tire sensitive skin. Consult a physician for serum selection and use suited to you.