A line with a - in front of it will not make it to the new file.

A line with a + in front of it is not in the old file.

A line with no sign is in both files.

Ignore the wording:

If you want a - line to make it to the new file, delete the - but carefully leave an empty space in its place.

If you want a + line to not make it to the new file – just delete the line.

What could be simpler?

Don't forget to change the two pairs of numbers at the top so that, for each pair, the number to the right of the comma is exactly equal to the number of lines in the hunk for its respective file, or else the edit will be rejected. That was too much of a mouthful so they didn't bother explaining it.