The Greek word for fraternal or brotherly love, as distinct from agape and eros. It arises from friendship and familiarity between two people or between a person and a thing (a pet, a house, a country). It implies a level of intimacy and comfort with each other, as between two best friends or longtime roommates.
Philos is not a "lesser" form of love than agape, but it is "easier". It means a level of true comfort in the presence of another person, of understanding and trust. It's not a prerequisite of agape, either; you can have agape love for someone you don't trust or even like, and vice versa. But all three are necessary components of a true romantic marital relationship.