A traditional French nursery round that Jean-Luc Picard sang as a boy and uses to keep three frightened children calm while they climb out of a wrecked turbolift in Disaster.
Frère Jacques is an old French children's song, sung as a round. Jean-Luc Picard, who grew up in La Barre in France, knows it from childhood.
In Disaster (2368), a quantum filament cripples the Enterprise-D while Picard is showing three science-fair winners, Marissa, Jay Gordon and Patterson, around the ship. Trapped in a turbolift with a broken ankle, he makes the children his crew and leads them up the shaft, and when they falter he gets them singing Frère Jacques, the one song they all know.
The tune returns in Lessons the following year, when Picard and Commander Nella Daren play together for the first time and he offers it on his Ressikan flute as a simple melody for her to build on.