"Missing the timing"
Sometimes an optional effect can only be activated "when" a condition happens. In this case, you are only allowed to activate the effect when the activation condition being met was the last thing to happen in the game. If something else happened after the activation condition was met, then you have missed the timing, and cannot activate the optional effect. Optional effects which can miss the timing are usually phrased "When" or "you can"
This mainly applies to Monster Cards with optional Trigger Effects such as Peten the Dark Clown and Poison Draw Frog, but it can also affect Spell and Trap Cards such as "Appropriate" and "Heart of the Underdog".
Some examples:
Example #1
If Peten the Dark Clown is Tributed for a Tribute Summon, the last thing that happened was that a monster was Summoned, so the effect of "Peten" does not activate.
This is because you first Tribute Peten the Dark Clown as a cost, and afterwards the other monster is successfully Tribute Summoned. The last thing to happen was not Peten the Dark Clown being sent to the Graveyard, so it misses the timing.
Example #2
If Peten the Dark Clown is Tributed for Mystic Wok or Cannon Soldier, the last thing that happened was that those effects resolved, so the effect of "Peten" does not activate.
This is because you first Tribute Peten the Dark Clown as a cost, and afterwards "Mystic Wok" resolves and increases your Life Points. The last thing to happen was not Peten the Dark Clown being Tributed, so Peten the Dark Clown misses the timing
Optional effects that do not miss the timing:
Note thatMissing the timing only applies to optional effect that are activated "When" or "you can" something happens, It does not apply to optional effect that are activated "If." something happens, such as Watapon or Ultimate Ancient Gear Golem, and it does not apply to optional effects that are activated "After.." something happens (like Voltanis the Adjudicator), Effects like these can never miss the timing, Similarly phrased "When" compulsory effects also cannot miss the timing.
Hope you have learned something here today that you can take into your future duels <3