So far, all the rhythm patterns you have played have involved strumming all the notes of the chord (five or six strings). Bass note picking involves playing a bass note from the chord, followed by strumming the first three strings. Bass notes are the notes on the fourth, fifth or sixth strings.
For example, in Exercise 4, hold a G chord and pick the sixth string (indicated ), then strum the first three strings three times. Always hold your left hand fingers in position when you are bass note picking.
Exercise 6 is in ^ time. The bass note is followed by only two strums.