What is the meaning of [Not good is it to harp on the frayed string]

Don’t keep on talking about the same thing or bringing up the same subject.
Literally ‘to harp’ is to play the harp; figuratively ‘to harp on’ is to dwell
tediously on a matter. Here ‘frayed ‘means that the harp-string (and the
subject) is becoming worn out through too much use.

##### See also:

* Don’t take your harp to the party
* Harp not for ever on the same string
* It is useless to flog a dead horse