A dream guide is any dream character who helps the dreamer with exploring the dream world and with controlling his/her dream. A dream guide can be anyone or anything that helps you when you are (lucid) dreaming, by getting you out of a dangerous situation, delivering you from pain, having you become lucid or showing you how to control your dream.
Dream guides can be really helpful when dealing with nightmares.
A person can have a single dream guide for all his/her dreams, several dream guides that show up randomly, or even one different dream guide at each dream. Also, the presence of a dream guide in dreams can be taken as a dream sign.
Finally, one can project their own subconscious into the dream guide, and so being able to talk to their inner self. This is useful when dealing with emotional problems, traumas, and similar issues.