Transaction validation is the process of determining whether or not a transaction complies with specific rules in order to be considered valid.

To validate the hash algorithm giving rise to a Blockchain transaction the input needs to be guessed and checked. Even a small minimalistic change in the input can have catastrophic changes in the output (butterfly effect).
This is where the consensus mechanism kicks in.

What is Validating a Blockchain transaction?

For every transaction that takes place on the Blockchain a bunch of people known as validators check and verify each transaction. This is called the Proof of Work (PoW) mechanism where every work done on the Blockchain requires proof. The calculation takes time and once the right numbers are matched then the block is said to be solved and verified or in simple terms, the block is “mined.”

Anyone can mine and verify transactions, and there exists a reward for participating. On Jan 12 2009 Hal Finney received the world's first bitcoin mining reward (10 BTC) for mining the block-70. Presently the block reward is 6.5 BTC per block mined.

It is also important to note that in a Blockchain every block has a certain hash which is derived from the hash value of the previous block and the block that follows has a certain value derived from the present block.
This ensures a solid chain and changes to the hash value of one block will lead to the changes in hash values of all the blocks in the Blockchain hence the need for validators in the first place.

Some extra info just for fun:
-In the Bitcoin Blockchain, a new block is added once every 10 minutes
-In Ethereum the time taken is 15 seconds
-While in Solana a new block is generated every 400ms!

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