There are plenty of scales and plenty of modes. This app ships with the so-called "church modes". These modes are mostly used in popular music in the western world.
A scale is just a sequence of tones with different distances. A mode is built up by creating chords (by layering thirds) starting on each tone of the scale. So you could say a scale is the base of a mode.
By doing this with the church scales, you will create seven different chords. These chords will be highlighted in the circle of fifths.
There are more than the church modes. You could build up modes on different scales, but as this is not very common, this app only includes the "church modes" but can be extended with more than 80 plain scales which will not be available in "Modes" and "Songwriting" mode of the app.