When something is so commonplace you know it well, it's familiar. A familiar song might be one that you hum along to without even thinking about it.
Like family, anything or anyone you're familiar with has been in your life for a while. It can be used in many ways. After six years of Spanish, you'll be familiar with how to conjugate verbs. You and your close friends are on familiar terms. The smell of musty tent fabric is familiar to anyone who grew up going camping a lot, and may bring on a wave of nostalgia.