Why random words work
The strength comes from the computer picking the words, not from the words being clever. Each word from a list of 7,772 adds almost 13 bits. Five words give about 64 bits, six about 77 bits.
Words you choose yourself, like song lyrics or a favourite quote, are far weaker, because people pick predictable phrases.
When to use a passphrase
Use one for passwords you have to type by hand: your password manager’s main password, your computer login, or a Wi-Fi network guests need to type in.
Word list
The English words come from the EFF large word list (CC BY 3.0), made for exactly this purpose. Everything happens in your browser; the passphrase is never sent anywhere unless you put it in a temp.cool link.