This word leannoth seems to point to some kind of instrument unknown (Ps. 88, title). The whole phrase has by others been rendered, “On the sickness of affliction: a lesson;” or, “Concerning afflictive sickness: a didactic psalm.”
Easton’s Bible Dictionary
Mahalath Leannoth Maschil
Easton’s Bible Dictionary (1897), public domain. Text via Theographic Bible Metadata, CC BY-SA 4.0.