Each block in a blockchain has a unique signature. A fingerprint.
When explaining what a blockchain block is we produced the signature using the SHA256 algorithm
There are other algorithms like MD5 that are commonly used to identify anything digital.
For example suppose I send you a file. How do you know that the file was not tampered with in transmission?
I could also send you the signature. It will be a long number like:
2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824
That's the signature of hello using the SHA256 encryption algorithm.
You can try it yourself here:
If you type hello into the field you will get the signature shown above.
So with that we can digitally sign anything. When we share the signature we are proving that what we are sharing was not tampered with.