Most modern songs have an underlying sequence of chords. Your song starts with the chord A minor, quickly goes to E minor, mixes in a C major chord, etc. Normally you play the notes 'by hand', for each chord individually. The quality of your progression and as a result the quality of your song is limited by your ability to play these chords, and your time to iterate and try different arrangements. Sometimes finding the right notes can take forever and often you end up with something that sounds nice but also not very special.
The twist in MelodyBuilder is to detach the way you play a chord from the chord itself. In MelodyBuilder you create so-called patterns, templates that describe how to play any chord and you can apply these to any chord. Create a pattern, e.g. for a bassline once, and apply the very same pattern to you whole chord progression and it automatically will play the right notes. It's a bit like a classic arpeggiator but much more powerful. This puts you into the comfortable position of only having to care about what your chord progression is and not what notes the individual chords consists of. And for this, MelodyBuilder comes with a huge chord database that allows you to create chord progressions you never thought of before.