Finality in the world of cryptocurrency & greater finance is one of the fundamental properties of sound money in trade and commerce.
Finality is a measure of the time between the submission of a transaction and its completion. More than that, finality specifically refers to the state of a transaction (after completion) where it is considered irreversible/immutable.
example of finality in blockchain:
Finality in Bitcoin is widely accepted as being 6 blocks (10min a block), so after 1 hour a bitcoin transaction is institutionally considered to be final.
example outside of blockchains:
Finality is the greatest value proposition with Fiat/Cash money for the reason that it is instant; once you physically hand over the cash a transaction is arguable instantly considered finalized.