What's this on a single line diagram? What's its function?

It is a neutral grounding transformer with a secondary loading resistor to effect high-resistance grounding of that big (1220MVA) synchronous generator. It connects from the neutral (star point) to earth. The drawing is incomplete, it doesn't show the neutral side of the primary being connected to earth but it is in reality.

The zero sequence reactance of synchronous generators can be significantly smaller than the positive and negative such that phase-ground faults can be significantly higher than 3-phase faults. So, we restrict the current that can flow in the neutral.

Below is a three-line view of what it looks like. The 59N is a fundamental frequency neutral overvoltage relay that is used to detect phase-ground faults from the terminals (and buswork) down to within about 5-10% of the neutral.

Below is a picture of one of these neutral grounding transformers (distribution class) along with the secondary loading resistor (this is from a nuclear unit about the same rating as your machine).