In Magic the Gathering, the 'stack' is the imaginary area where spells and abilities go when they are cast/played, and are waiting to resolve.
So say one player casts a Lightning Bolt, targeting one of his opponent's creatures. He pays the mana cost and the spell goes on the 'stack'. The opponent however to save his creature, responds to this by casting 'Giant Growth' on that same creature. The stack goes by 'LIFO' order - last in, first out. It means that the latest object to go on the stack, will be the first to resolve. So Giant Growth will resolve first, and then Lightning Bolt. But say if Player A had some kind of counterspell, he could place that on the stack above Giant Growth to counter it and remove it from the stack, meaning ultimately the creature will die to Lightning Bolt.
Tapping permanents to generate mana (be they land or anything else that can do it), a 'mana ability', does not use the stack, it is uninterruptable so can't generally be countered. Morphing, flipping a face-down Morph creature face-up, also does not use the stack, so a Morph cannot be quickly dispatched once the intention to flip is known.
The stack can be interrupted as it resolves. Imagine a stack with objects A, B and C on it. When C resolves, either player might feel the need to play something else, so this will become object D on the stack above A and B, and will resolve before them.
No, Riot does not stack in Magic: The Gathering.
Yes, planeswalker abilities use the stack in Magic: The Gathering.
No, emblems in Magic: The Gathering do not stack. Each player can only have one emblem of a specific type at a time.
Yes, enchantments can stack in Magic: The Gathering, meaning multiple enchantments can be attached to the same permanent or player.
Yes, the mechanic "myriad" in Magic: The Gathering does not stack with other abilities or effects.
Yes, hardened scales do stack with other 1/1 counters in Magic: The Gathering.
No, the flanking ability in Magic: The Gathering does not stack with other instances of flanking on the same creature.
Enchantments in Magic: The Gathering can stack if they have different names and do not have conflicting effects. Multiple enchantments with different abilities can be on the same creature or player at the same time.
Yes, Morph utilizes the stack in gameplay mechanics in Magic: The Gathering.
No, double strike effects do not stack in Magic: The Gathering. If a creature already has double strike, giving it another double strike effect will not have any additional impact on combat.
You can play a sorcery card in Magic: The Gathering during your main phase when the stack is empty and it's your turn.
Yes, in the game of Magic: The Gathering, players can prevent or mitigate damage on the stack by using instant spells or abilities that can counter or redirect the damage, or by using cards that provide protection or increase toughness of creatures on the stack.