"Password cracking" is when somebody works out what a password is. This demonstration is a visualisation of the most basic way that hackers can "crack" passwords: using computers to try every password they can.
Computers can do this very, very quickly. Well secured websites make this difficult and can slow it down to only thousands of guesses per second, but it can be billions of guesses per second! Usually, hackers do this with a big list of "hashed" (gibberish) passwords from a company that has suffered a data breach.
How to use the demonstration:
Key Takeaway: Safe passwords MUST be strong enough to avoid being cracked by this most basic approach as a bare minimum.
In reality, passwords also MUST be unpredictable. Better password cracking techniques will check passwords that are common alterations to known words (e.g. swapping S for $, or putting a number on the end) and previously breached passwords. Predictable passwords will be cracked in seconds.