I think the word you are looking for is tooltip. There was a similar report recently:
In Tooltips no-show - Ubuntu 17.10 the exact same problem as yours was reported. The suggestion was to check the gtk style settings. The original reporter never got back to us if it helped.
In Menu bar issue / no tooltips showing it was reported that there where no tooltips at all. It seems to be the case that here changing the style settings for gnome/gtk under KDE where to blame. Here we got conformation that this fixed the issue.