In the context of blockchains, finality refers to the point at which a transaction can be considered final and irreversible. There are two main types of finality; probabilistic and deterministic.
Probabilistic finality means that finality is guaranteed probabilistically, and thus, in theory, the transaction could be reversed at any point. However, doing so becomes progressively difficult and expensive. For most blockchains providing probabilistic finality, there is a recommended number of blocks to be added following the transaction until the transaction can be considered complete. For example, it is recommended to wait until six additional blocks have been added to the Bitcoin blockchain before considering a transaction to be final. Blockchains that use a form of Nakamoto consensus, such as Bitcoin, typically have probabislitic finality guarantees.
Determinstic finality means that once a transaction is considered final, there is no way for it to be reversed. Blockchains that use a form of Classical or Hybrid consensus typically have determinstic finality guarantees.