For example, this is the FEN for the starting position of a chess game:
rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
You can copy a FEN from most places on chess24 (such as Live Tournament broadcasts) in the Analysis tab.
Here's the final position from the final decisive game of the 2014 Carlsen vs. Anand World Championship match:
8/4b3/4P3/1k4P1/8/ppK5/8/4R3 b - - 1 45