Genbutsu literally means "the real thing", a tile that a specific opponent has already discarded and that therefore sits in their kawa. Because furiten bars that player from winning by ron on one of their own discards, the tile is certain against their ron. Complete public constraints can sometimes rule out every remaining shape, but merely counting the other copies of one tile is not always enough: a run or special shape may wait on a tile without already holding a copy.
You read genbutsu straight from the discard pile. A tile is genbutsu against a player if they discarded it themselves, or if anyone discarded it after their riichi and they let it pass, since a missed ron under riichi locks them into furiten for the rest of the hand. Note that it is safe only against that one opponent and only against ron, never against tsumo. This is the foundation of betaori, the discipline of folding.
When you decide to fold, you chain genbutsu discards to get through the hand without dealing in. Genbutsu is safer than suji or wall reads, which only lower the odds. A common pitfall is treating a tile as safe against the whole table when it is safe only against the player who discarded it; against a second opponent who is also tenpai it can still be a live deal-in. Always track safety opponent by opponent.