What does the T symbol signify in electrical diagrams?
In the diagram in the original post

 The bar symbol is analog ground (in this case).  

 The triangle stands for analog ground.  

 The lollipop is the Vdd supply rail.

However, this does not follow the normal schematic convention.  That's why that notation is confusing, and we have to tilt our heads and guess from the context.

Standard convention

The following symbols are roughly ordered by decreasing voltage.

 Upright T, or upright lollipop (not upside-down, not on the side).  Positive supply rail.

  Signal ground.  Sensitive ground.

 Digital ground.  Less sensitive ground.

  Upside-down T, or upside-down lollipop.  Negative supply rail.