From Famitracker Wiki
0xy is an effect that activates a crude arpeggiator. It can be used on any channel except DPCM. x and y are offsets in semitones from the base note. x = second note, y = third note. x and y are measured in amount of semitones higher than the note in the note column, or "0". It is possible to omit the y parameter to achieve a faster arpeggio between two notes, i.e., using 0x0 instead of 0xy. Effect 000 or pitch slide effects will stop the arpeggio.
The effect works by rapidly alternating between three (or just two) notes, which creates an illusion that these notes are playing simultaneously. Arpeggios (or "arps" for short) can be used to play "chords" without sacrificing two or more channels. Historically, they have been more often used by western composers (specificially European ones) than eastern composers. Jeroen Tel (Robocop 3), Jonathan Dunn (Jurassic Park, Robocop 2), and Tim & Geoff Follin (Silver Surfer) are examples of such composers who used arpeggios extensively.
C-4 00 F 047
C-4 is note, 00 is instrument number, F is volume. Will play arpeggio chord C major (C, C+4=E, C+7=G) until a new 0xy effect (such as 000) or pitch slide effect is set.
C-4 00 F 070
C-4 is note, 00 is instrument number, F is volume. Will play arpeggio chord C5 (C, C+7=G) until a new 0xy effect (such as 000) or pitch slide effect is set.
Chord loop — sequence of four chords (C,A,F,G) in single channel using arpeggio