The "R" signifies that it is a held over beat from the previous measure, so a rest of sorts, but you let it ring. Essentially, the & of the 4 beat in the 4th measure is getting ahead of the beat of the song and then you sit on it for a beat to get back in time. The Main Riff Generally, this will be the double upstrokes near the 2 and 4 beats of the measure, it's syncopated behind the beat in a standard ska sort of way. Listen to the song, it's not hard to figure out. you F#m G (azchords.com)