Sometimes a Trigger Effect says that "when" a condition happens, you "can" activate its effect. In this case, you are only allowed to activate the effect when the condition being met was the last thing to happen in the game (activating cards & effects that haven’t resolved yet doesn’t count).
If the triggering condition happened for an optional Trigger Effect, but something else has happened after that, then you have "missed the timing" and you cannot activate it. For example, this can happen if the optional Trigger Effect monster was Tributed for a Tribute Summon or to activate a card effect, or if the triggering condition happened in a chain and wasn't Chain Link 1, or if another card effect or game effect has happened since then. The triggering condition has to be the very last thing that happened in order for a "when... you can" optional Trigger Effect to activate.
Note that this only applies to effect that state "When...you can...". It does not apply to effect that state "If...you can..." like "Watapon" or "Ultimate Ancient Gear Golem", and it does not apply to effects that state "After...you can..." like "Voltanis the Adjudicator". Effects like these can never miss the timing.
For some good specific examples of "missing the timing", see the rulings for "Pinch Hopper", "Archfiend of Gilfer", "Butterfly Dagger - Elma", "Trap Hole", "Heart of the Underdog", "Peten the Dark Clown", or "Dark Magician of Chaos".
Types of Monster Cards
Types of Spell Cards
Types of Trap Cards
Other Types of Card
ATK - DEF - Level - Attribute - Type
Playing Field Zones
Types of Summon
Types of Monster Effects
Types of Effect Monsters
Types of Spell Speed
Aqua - Beast - Beast-Warrior - Dinosaur