Tremolo picking is a guitar picking technique where you pick a note repeatedly as fast as you can, and is commonly used in music featuring electric guitars.
Tremolo picking is identified on sheet music by additional strokes through the stems of the note. Three strokes represent whole, half and quarter notes; while two strokes represent eighths; and one stroke represents sixteenths.
For beginners, the best way to learn tremolo is slowly, staying on beat and keeping time to lay a foundation for faster speeds in the future.