The Summon response window is an unofficial term for the activation timing of cards that respond to the Summon of a monster(s), such as "Bottomless Trap Hole", "Trap Hole", etc. There is up to one chain in the "Summon response window"
During the Summon response window, due to Fast Effect Timing, first the turn player must activate any compulsory effects that they triggered, such as "Summoning Curse" and "Summoner Monk", then their opponent must do the same. Then, the turn player has the opportunity to activate optional effects like "Torrential Tribute" and "Armageddon Knight"; if the turn player passes, then the opponent also has the opportunity to activate these effects. If at any point either player starts a Chain, standard Chain rules are followed.
This window will open even if a monster is Summoned by a card effect rather than as an inherent Summon. However, this timing will be missed if a monster(s) are Summoned by a card effect that is Chain Link 2 or higher, unless the activation of all other cards and effects that would resolve after it are negated. Also, if the card effect that Special Summoned the monster(s) did something else immediately after Special Summoning the monster (e.g. "Magical Dimension"), then this timing will be missed.
Unlike the Summon negation window, cards that do not specifically respond to the Summon can still be activated (as long as their timing is correct). Also, cards that respond to the Summon specifically can be activated in later Chain Links, since they are responding to the action and not negating it.
During this window, the monster(s) have already been successfully Summoned, so are on the field. If the Summoned monster(s) have Continuous Effects, these are now active. If monster(s) that were just Summoned are moved to a different location during this window, they are considered to have left the field (unless returned to the Main Deck).
Furthermore, if the card effect that Special Summoned the monster did something else immediately after Special Summoning the monster (e.g. "Magical Dimension", "Drill Warrior", "Telekinetic Power Well", "Trojan Gladiator Beast", etc.), then this window does not occur and any cards that can be activated in response to a Summon miss the timing.
The turn player attempts to Summon "Black Luster Soldier - Envoy of the Beginning", banishing 1 DARK and 1 LIGHT monster from his/her Graveyard. The Summon was not negated, so the Summon response window is now open, and the turn player can activate cards in response to the Summon of a monster. If the turn player passes doing so, then the opponent may do so.
If both players pass without responding to the Summon, nothing happens and the game state becomes open again.