Update: the favicon is now 'XLM', the currency symbol for Lumens:
What do the letters 'sdf' in the old favicon
The letters 'sdf' stand for Stellar Development Foundation, the organization which maintains the Stellar network. Why they are in lowercase can probably only be answered by the Community Managers (employees of Stack Overflow, who maintain this site).
'Stellar' itself would be too long to fit; the risk of using just 's' is duplicating another (possibly future) site in the Stack Exchange network.