A stack is a pile where you only touch the top. You add items on top with push and take them off the top with pop. The bottom stays untouched until everything above it leaves.
LIFO means last in, first out. The most recently added item is the first one you get back. Think of a stack of plates. You wash the last plate you set down, not the first.
This ordering matters when you must reverse or backtrack. Browser back buttons, undo history, and function calls all rely on it. The newest work gets resolved before older work waits underneath.
Rewriting in plainer words…
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