The Fisher Transform is a clever bit of maths that takes ordinary price movement and re-shapes it so that turning points stand out sharply — a bit like turning up the contrast on a blurry photo until the edges become crisp.
Normal price tends to wander around gently, which makes tops and bottoms genuinely hard to spot until after they have already happened. The Fisher tool deliberately exaggerates the extremes, so when it hits an unusually high or low reading, it is waving a flag that a sharp reversal could be close — and it often does this a little earlier than a standard speedometer like the RSI would.