The scale lock button displays the current scale and locks the keyboard so that only melody notes of the scale can be entered.  Click on the button to lock / unlock the scale.

It is a function for the more theoretically inclined among you.

Scales are selected with the Scale menu option and displayed by means of scale balls.  The default scale when starting the program is the major scale.  The other scales available via the Scale menu option are shown below: 

Major and minor scales are characterized by a distinct pattern of half steps (HT) and whole step (WT) intervals.  And the balls reflect this pattern.  When the scales are locked, only the notes below a scale ball can be selected and played.

The location of the red scale balls matches the start of the particular scale corresponding to the current key signature.  For example the first red ball of the FMajor scale below starts on the F note, 

F Major

Here are the whole-step half-step patterns for the four scales currently available in FastChords:

Scale

pattern

Major

  WT - WT - HT - WT - WT - WT - HT

Natural Minor

  WT - HT - WT - WT - HT - WT - WT

Melodic Minor

  WT - HT - WT - WT - WT - WT - HT

Harmonic Minor

  WT - HT - WT - WT - HT -  w/H - HT

(the w/H symbol means a whole step plus a half step)

Note that for a given scale, major or minor, the particular notes enabled depends on the signature key, which in turn dictates the name of the scale.

Something fun to do is to turn sound ON and travel the keyboard with the left mouse button down to hear how different scales sound.