At the end of 2014's Transformers: Age of Extinction the leader of the Autobots, Optimus Prime was seen rocketing into space having learned from the bounty hunter Lockdown that his employers had contracted him to return Optimus to them. To many this ending seemed out of character for Optimus Prime, who has been shown in the movie series to be highly devoted to his comrades and friends and thus unlikely to abandon them.
According to the movie lore, 75,000 years BC the First Primes, introduced in 2009's Transformers: Revenge of the Fallen, sacrificed themselves to protect the Matrix of Leadership after stopping one of their own Fallen from activating the Sun Harvester and destroying all life on Earth. During the movie, Optimus is inferred to be a descendant of one of the seven Primes.
However, according to the original lore Optimus Prime is actually one of the original Primes, the last Prime who opted to be transformed into a lesser descendant AKA a Transformer and leader of the Autobot faction. The Primes themselves were created by an entity called Primus, who along with Unicron, the chaos-bringer was created by The One.
The revelation of the fifth movie's title suggests that the above lore is going to be further explored with the introduction of Primus and the likelihood of Unicron (pictured above) being teased as the villain for the sixth installment.
Production begins next month, with filming in Detroit expected to take place as with the previous installments, with Transformers: The Last Knight hitting theaters June 23rd, 2017.