The convert button converts melody notes to non-melody notes.  This helps identifying chord names from written music.  Notes are entered and converted one at a time.  The resulting chord name (if there is a match) appears in the chord name window.  For example, if the following notes are entered and converted, one at a time:

the following chord name results:

The convert function is not a mainstream function of FastChords.  Most often the name of chords is known and part of the written music.