To understand the report, remember that diff is prescriptive, describing what changes need to be made to the first file (file1) to make it the same as the second file (file2).
Specifically the d in 1do means delete and the a in 2a2 means add Specifically, the d in 1d0 means delete and the a in 2a2 means add.
Thus:
1d0means line 1 must be deleted infile1(applesapples).0in1d0means line 0 is where they would have appeared in the second file (file2) had they not been deleted. That means when changingfile2tofile1(backwards) append line 1 offile1after line 0 offile2.2a2means append the second line (oranges) fromfile2to the now second line offile1(after deleting the first line infile1,orangesswitched to line 1)