Short answer: a fair value gap is a stretch of price that was crossed so fast only one side traded it. On a chart it is a three-candle pattern where the first candle's high and the third candle's low (or the reverse) do not overlap. Price often comes back to "fill" that gap before moving on.
Look at any big candle. Compare the candle before it and the candle after it. If the wick of the first and the wick of the third leave a space between them, that space is the gap. A bullish gap sits under price after a rally; a bearish gap sits above price after a drop.
Inside the gap, almost no trading happened in one direction — there were buyers but no sellers, or the other way round. When the move slows, price often drifts back into that thin area to find the trade it skipped. A gap does not have to fill completely; touching its middle is common.
A gap is most useful when it sits next to something else — a swing level, an order block, or a key retracement. On its own it is just a place price may visit.
See gaps drawn on real charts in the Sylvo market reads.
No. Many do, but gaps in strong trends can stay open for a long time. Treat a gap as a place price may return to, not a promise.
An order block is where large orders were filled before a move; a fair value gap is the stretch the move crossed too fast to trade. They often sit right next to each other.
Gaps on the 1-hour, 4-hour and daily charts are more meaningful; on very short timeframes they appear and fill constantly.