Faithful describes someone who is reliable and consistent. If you're a faithful volunteer at the animal shelter, you show up every Saturday and work hard to help the animals.
A faithful friend is loyal and steadfast — it's no wonder that many people consider their dogs to be their most faithful companions. Though faithful usually describes a person, you can also use it to talk about a particularly reliable object, like your faithful hiking boots. There's another sense of faithful that means "remaining true to," like faithful believers who pray and attend religious services regularly.