This word leannoth seems to point to some kind of instrument unknown ( Psalms 88 , title). The whole phrase has by others been rendered, "On the sickness of affliction: a lesson;" or, "Concerning afflictive sickness: a didactic psalm."

These dictionary topics are from
M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition,
published by Thomas Nelson, 1897. Public Domain, copy freely.