The “first line indent” is relative to the “before text indent.”
“Before text” set the space from the left margin(1) while the “first line indent” determines where to put the first line in reference to the “before text” setting. That is, if we define the “before text” indent as 1 cm and a first line indent of 0.5 cm, this first line will be found at (1 + 0.5) cm = 1.5 cm from the effective edge of the text area.
On the other hand, if we define a “before the text” indent as 1 cm and a first line indent of –0.5 cm, the first line will be (1 − 0.5) cm = 0.5 cm from the effective edge of the text area.
(1) Actually, because you can have a “space to content” from the margin, that’s not completely true, but anyway.