With the dns-01 challenge, you prove to the CA that you are able to control the DNS records of the domain to be authorized, by creating a TXT record with a signed content.
Dns01Challenge provides a digest string:
Dns01Challenge challenge = auth.findChallenge(Dns01Challenge.TYPE); String digest = challenge.getDigest();
The CA expects a TXT record at _acme-challenge.${domain} with the digest string as value.
The challenge is completed when the CA was able to fetch the TXT record and got the correct digest returned.