The tool tries every possible combination of characters until it finds a match.
The famous rockyou.txt wordlist contains 14 million real-world passwords. ntlm-hash-decrypter
The search results are often confusing. You will find dozens of websites claiming they can instantly "decrypt" any NTLM hash for free. You will see tools like John the Ripper, Hashcat, and Responder being mentioned in the same breath. The tool tries every possible combination of characters