In music notation, A Tie is a curved line over or under two or more notes that "ties" the two notes together into one.

If the second note of a tie is the same as the first note, then the second note is not played, rather, the first note continues playing for the duration of the second note.

Use the  button to mark a note as a tie note.

FastChords checks whether the preceding melody note in the chords table is the same note as the one you are attempting to mark as a tie note.  If it is not, an error message will alert you of that fact.