Short answers, pulled from the story.
The Force is a metaphysical energy field described in Star Wars as interconnecting all living things and maintaining cosmic balance. Force-sensitive characters use it to access abilities including telekinesis, mind control, precognition, and superhuman strength. It is divided into a light side, drawn on by the Jedi for selfless and peaceful ends, and a dark side, used by the Sith for selfish and destructive goals.
Lucas developed the Force by distilling what he called "the essence of all religions", aiming to suggest belief in God without endorsing any specific faith. He drew directly on a line spoken by Canadian cinematographer Roman Kroitor in Arthur Lipsett's 1963 National Film Board film 21-87. New Age concepts such as qi were also widely circulated in 1970s San Francisco, where Lucas lived while writing the early drafts.
Midi-chlorians are microscopic creatures introduced in The Phantom Menace (1999) that live inside all living cells and connect characters to the Force. Their introduction was controversial because many fans felt the Force had been reduced from a spiritual concept to a biological one. Critic Evan Narcisse wrote in Time that midi-chlorians ruined Star Wars for him and for a generation of fans because "the mechanisms of the Force became less spiritual and more scientific".
The Force has been compared to Zoroastrianism for its sharp division between good and evil, to Taoism for the interplay between its light and dark sides, and to concepts including prana, qi, and a Navajo prayer. The Christian valediction dominus vobiscum, meaning "the Lord be with you", directly influenced "May the Force be with you". Jennifer Porter, professor of religious studies at the Memorial University of Newfoundland, described the Force as "a metaphor for godhood".
"May the Force be with you" is a Jedi blessing expressing hope for others and was modeled on the Christian phrase dominus vobiscum. In 2005, the American Film Institute ranked it number 8 on its list of the 100 greatest movie quotes. May 4 became Star Wars Day as a pun on "May the Fourth be with you".
In 1985, President Ronald Reagan said "the Force is with us" while promoting the Strategic Defense Initiative, a missile defense program widely nicknamed "Star Wars". Some weeks earlier, Reagan had compared the Soviet Union to the Galactic Empire. Scholars noted that Reagan's phrasing reversed the original blessing's other-focused ethos, turning a hope for others into a possessive assertion.