The shape is a vertical line with a circle, like a bulb, on the bottom. The shape represents a thermometer.
Originally the temperature icon was a vertical line with a bulb shape on the bottom and a little flag or nub on the top right. It was part of a system that included hot, cold, and medium. The hot icon was a vertical line with a flattened bulb on the top, cold was a vertical line with a flattened bulb on the bottom and medium was a vertical line with a the flattened bulb in the middle. The hot and cold broke the orientation rule (tactile icon should be unique and recognizable regardless of the orientation) and medium was somewhat close, in a tactile sense, to the plus tactile icon. Also we had a request to create a tactile icon for music and the shape we wanted was very similar to the temperature icon. Sooo temperature was simplified, hot and cold are now more representative of their visual icons and we have a music tactile icon. A tactile icon for medium exists and it is logical but truthfully it needs something, probably more testing and more discussions with other designers to make me feel comfortable with it.
Note: my primary tester is not a fan of the new hot and cold tactile icons. The original temperature set was very simple. I can understand her view, especially since the icons are tested with the correct orientation on a table. This is where a design selection has to be balanced with the whole.