json-ld signatures with Ed25519VerificationKey2018

Hi all,
I am experimenting with json-ld signatures, using digital bazaar's library 
https://github.com/digitalbazaar/jsonld-signatures

I notice when this algorithm is used Ed25519VerificationKey2018 the library
crates a jwt with the following header:
header = {
    "alg": "EdDSA",
    "b64": False,
    "crit": ["b64"]
}
I am struggling to understand what is this header and why is there. Anybody?

Thanks,
Nikos