Hello Josh. This means there is a 7th above the root present.
For example...
We play our C Harmonica commonly in the Key of G (2nd Position/Cross Harp). The I Chord would be G B D. G is the root... the bottom of the chord. B is the 3rd, since it's the 3rd from the root (G A B). D is the 5th, since it's the 5th from the root (G A B C D).
When it reads as I7, then add a 7th from the root, which is F# (G A B C D E F#). One change that is assumed, is that the 7th is a minor 3rd away from the 5th (three half steps). This would make the F# into F, which is a more bluesy sound.
Hello Josh. This means there is a 7th above the root present.
For example...
We play our C Harmonica commonly in the Key of G (2nd Position/Cross Harp). The I Chord would be G B D. G is the root... the bottom of the chord. B is the 3rd, since it's the 3rd from the root (G A B). D is the 5th, since it's the 5th from the root (G A B C D).
When it reads as I7, then add a 7th from the root, which is F# (G A B C D E F#). One change that is assumed, is that the 7th is a minor 3rd away from the 5th (three half steps). This would make the F# into F, which is a more bluesy sound.
My complete lessons on understanding chords is found in Music Theory Study 3 (https://www.bluesharmonica.com/lessons/music_theory_study_3_intervals_ch...). If you haven't done Music Theory Study 1 and 2, make sure to start there first.