Help digesting schematic layout
You've asked two questions:


  Can someone help me figure out what the little letters and numbers mean on the pins of this ic? 


As Mike F. has mentioned, my answer to this previous question also answers this part of your question - those are the BGA ball numbers on the IC.


  They're used throughout various ics in this iPhone 6 schematic. Are they indicating sequence?
  
  For example A1 happens first then A2 etc or?


They are indicating the order (layout) of the balls on the IC, but not what happens functionally. For example, across the top of IC U1401 in your image, the pins A2 B2 D2 C2 are shown as connected together, so they are all used at the same time.

Note that there are a variety of different BGA layouts, so you should check the manufacturer's datasheet to confirm the specific ball numbering for an IC. Based on available photos, the device shown as U1401 might be manufactured by Texas Instruments, however its datasheet does not seem to be publically available. 

Just as an example, here is a diagram from Texas Instruments showing the magnified layout for one of their other BGA packages (larger than U1401 in your schematic):


Image source - link

Notice that the circles for each "ball" are not complete; that is a convention indicating that this is the top (not the bottom) view of the package (confirmed by the text also saying "top view"). You can see that the upper row of the grid layout is row A and the left column is column 1. Then the row letters and the column numbers increment, producing the "sequence" like A1 A2 etc. that you noticed on that schematic.