This is a metaphor. When we take a bath and we are very dirty, the particles of dirt washed from our bodies float on the surface of the water. When we pull out the bath plug, the water level descends, and a 'ring' of dirt can often be seen clinging to the sides of the bath ('bathtub' in US English). It is a residue which tidy people remove, and its occurrence is a sign that the bath user should wash more often (or works in a coal-mine). Too much glee, asserts Mary Poppins, leaves an undesirable residue in the brain. The following line confirms this: "Take that joy and send it down the drain". The washing metaphor, and references to baths and mental 'cleanliness' or 'tidiness' occur repeatedly in the song, e.g. "For intellect can wash away confusion".