Split Second is a keyword ability.
It is a mechanic from the Time Spiral Block that states; "While this spell is on the stack, players can't play spells or activated abilities that aren't mana abilities". This gives the caster a good tactical advantage, meaning that the spell mostly cannot be countered or responded to. A good example of the Split Second mechanic is the Time Spiral card, Sudden Shock, which is just a version of the Core Set card Shock with Split Second.
However, this is not to say Split Second spells are entirely immune. Within the same block, there are creatures with Morph. Turning cards face-up through Morph does not use the stack and is not an activated ability (see rule 702.34d). For example, you could turn a facedown creature faceup triggering an effect in response to the spell with split second being played. Of course, you can still use mana abilities, as well, so if you're about to lose something that generates mana, you're still able to get that mana in response. (This is because Wizards of the Coast actively stays away from preventing mana abilities, considering how crucial they are to gameplay.)
702.60. Split Second
- 702.60a Split second is a static ability that functions only while the spell with split second is on the stack. “Split second” means “As long as this spell is on the stack, players can’t cast other spells or activate abilities that aren’t mana abilities.”
- 702.60b Players may activate mana abilities and take special actions while a spell with split second is on the stack. Triggered abilities trigger and are put on the stack as normal while a spell with split second is on the stack.
- 702.60c Multiple instances of split second on the same spell are redundant.