Diatonic is a word of Greek origin meaning "progressing through tones." Diatonic scales are the foundation of Western harmony. The basic major and minor scales are diatonic scales.You may wish to create a musical statement within a bass line that involves only the notes that are proper to the prevailing diatonic key of the moment and without any chromatic alteration. This creation would be diatonic in nature. There are 8 notes in the diatonic scale, the 1st and 8th degrees being the same "note" an octave apart. The 8th degree is an octave higher than the first note and has the same letter name.
A basic arpeggio is derived by skipping to every other note of the diatonic scale in order to create different intervals called chords when played simultaneously, and arpeggios when played individually. The tones of an arpeggio are typically not ringing together but are played one after the other.
Learn the diatonic major and minor scales and explore their arpeggios and then combine them while playing in order to create interesting and evolving bass lines. There will be more focus on learning and combining many other scales such as the blues scale, the major pentatonic scale, the different minor scales and modes, and their associated chords and arpeggios in future TrueFire courses.