An innkeeper is someone who owns or manages an inn. When you arrive at an inn, the innkeeper might be the person who checks you in and gives you a key to your room (and maybe a chocolate on your pillow).
The earliest innkeepers ran inns in Europe during medieval times. These inns provided lodging, food, and a place for travelers to leave their horses, and an innkeeper managed all of these details. Today in the UK, some pubs call themselves inns, so you might find an innkeeper serving pints there, while in the US an inn is almost always a more charming version of a motel, run by an innkeeper.