What does the “5D’s” mean in Yu-Gi-Oh! 5D’s?
The 5D's is meant to represent the legendary 5 Dragons (Signer Dragons) that the protagonists (Signers) use in order to fight the antagonists (Dark Signers).

These dragons are:


Stardust Dragon
Red Dragon Archfiend
Black Rose Dragon
Ancient Fairy Dragon
Black-Winged Dragon



   This was what the initial story made to believe anyway, because later on in the series, there is one more protagonist who awakens as a Signer and obtains a Signer Dragon; Leo obtains his after a duel where his life was on the line along with his sister Luna and Jack (both of whom were originally confirmed Signers). The sixth dragon is named Life Stream Dragon