Plain-text files that contain a large set of words or strings, typically usernames, passwords, common paths, used for:
Custom wordlists
Custom wordlist can be generated with the following tools.
Usernames
Passwords
Pre-made wordlists
For many wordlists see https://github.com/danielmiessler/SecLists.
Kali Linux
Pre-downloaded lists can be found in Kali Linux in /usr/share/wordlists.