Tibetan Buddhist Deities

Are Tibetan Buddhist Deities real (external)? I've been to two Medicine Buddha pujas and a Tsog offering and I can't figure out if it's all just hocus pocus. Professor Tom Yarnell's definition of Deity yogas as "ascertaining emptiness as arising in or experienced as the form of a deity" (not sure I got it right - it's from a Tibet House video ("what is Deity Yoga?")I'm trying to understand. Thanks!