I'm trying to connect to my atmega1284rfr2 using the basic application circuit in their datasheet. The red lines and letters are my own annotations.
I'm having trouble understanding what all the 
mean. Do they mean hook them up to positive supply? Ground? Dangling?
I get the impression that some should be dangling, but others should be bridged to ground, and others to the positive supply rail.
What do the T's mean?
Answer
The bar symbols are analog ground and the triangle symbols are digital ground.
They would be tied together somewhere (not shown), probably directly at one point but it's possible there would be something like a ferrite bead.
The orientation of the bars is immaterial (as with the triangle grounds).
This particular device has sensitive 2.4GHz RF circuits inside (probably on two dies), hence the extra detail devoted to grounding, but similar concerns are present whenever analog circuitry is mixed with digital. Here is a reference more oriented to the latter that goes into a lot of detail.
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