In guitar tablature, "r" typically stands for "release." When you see a notation like "15r13," it means you should first play the note at the 15th fret and then release (or pull-off) to the 13th fret in one fluid motion. This technique creates a legato sound, where the second note is produced without explicitly picking the string again. The release technique is commonly used in rock, blues, and metal guitar playing to create smooth, flowing passages.