What does the stack in Magic the Gathering mean?

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.