Cryptographic digest (or hash amount) has been calculated from this file using SHA256 algorithm and then saved in the blockchain. The original data is also now permanently saved.

What does it mean

It means that this confirmed blockchain transaction contains a file or a text, hash amount of which has been permanently stored inside transaction. It is now impossible to modify or backdate this data based on blockchain principle (link). If at later date you want to verify this data, and(or) you have a copy of it, you can paste it using form above, calculate its hash amount and compare with the hash amount stored in the blockchain. Matching hash amounts is a 100% probability that the data is authentic.
Hash function algorithm of SHA256 has been developed by NSA. More info http://en.wikipedia.org/wiki/SHA-2

How to make sure that data matches the signature

You need to calculate SHA256 hash amount and compare it with the one saved in the blockchain.

How to calculate SHA256 hash amount