A reaction is a triggered ability whose text contains the boldface "Reaction:" precursor. Always resolve a triggering condition before initiating any reactions to that triggering condition.
Unlike actions, which are resolved during action windows, each reaction may be initiated once each time its specified triggering condition occurs, as described in the reaction's ability text.
After a triggering condition resolves, a reaction window for that triggering condition opens.
Within the reaction window, the first player always has the first opportunity to initiate an eligible reaction (to this triggering condition), or pass. Opportunities to initiate an eligible reaction, or pass, continue in player order until all players consecutively pass, at which point the reaction window closes. Passing does not prevent a player from initiating an eligible reaction later in that same reaction window.
Once a reaction window closes, further reactions to that specific triggering condition cannot be initiated.
It is possible for multiple reactions to be initiated from the same triggering condition. Each reaction must resolve completely before the next reaction to the same triggering condition may initiate.
If multiple copies of the same reaction are eligible to be initiated, each copy may be initiated once against that triggering condition. Each eligible reaction may be used once per trigger. (If multiple copies of the same reaction are eligible, each copy may be used once.)