A Splice Effect (スプライス効果(こうか) Supuraisu Kōka) is a type of card effect only seen on Splice Spell Cards and Splice Trap Cards. Splice Effects are completely separate from a Splice Card's Spell/Trap Effect, and their lore box cuts into the card's artwork, much like the Pendulum Effect of a Pendulum Monster.
Splice Effects serve the purpose of initiating a Hybrid Summon, using itself and a monster affected by it in some way (being equipped by it, currently targeted for its Spell/Trap Effect, etc.) as the Materials. The Splice Effects may or may not have an extra effect relating to the intended Hybrid Monster, or an extra condition in order to even perform the Hybrid Summon.
Splice Effects, even those of Splice Trap Cards, can only be activated during the player's Main Phases, unless the effect allows it to be activated at a different time (such as the opponent's turn), and are treated as Spell Speed 1, meaning they cannot be chained to any effects or attacks.