- "All right, guys, uh, listen. This is a blues riff in 'B', watch me for the changes, and try and keep up, okay?"
A blues riff is a rhythmic pattern for the backup instruments in a band.
Since Marty McFly chose to play the song Johnny B. Goode, which had not been written yet, at the Enchantment Under the Sea dance, he gave the Starlighters brief instructions for accompanying him, so that he could sing and play lead guitar, thus "inventing" rock and roll.
Blues is a vocal and instrumental form of music based on the use of blue notes, which are played at a slightly lower pitch for expressive purposes. The style derived from spirituals and African-American work songs around the first decade of the 20th century, and was one of the influences of early rock and roll. 
A riff is a repeating, rhythmic pattern of notes or a progression of chords, that backup instruments can play when improvising a piece of music.
In B prepares the musicians to play in the key of "B" (although the song was actually played in the key of B flat).
Watch me for the changes was an instruction for the musicians to pay attention when he changed chords, and to try to change at the same time.
Try to keep up either meant that the band would have to learn quickly to follow in the new style or, it implied humorously, that Marty intended to play faster than the band was used to playing.
Use of the phrase
Marty turned away from the audience and spoke these instructions directly to the band. However, when he returned to 1955 for the second time, the voice of his other self on stage was as loud as if he had spoken the words through the microphone, even though his back was turned.