Bitcoin protocol allows a chain to be undone by a longer chain

Bitcoin protocol allows a chain to be undone by a longer chain

In Mastering Bitcoin, page 196, it says "While the protocol always allows a chain to be undone by a longer chain and while the possibility of any block being reversed always exists, the probability of such an event decreases as time passes..."

How and when will a chain be undone? How and when a block be reversed? Does Bitcoin core actually allow/support such operations?

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