What is unrestricted submarine warfare?

Unrestricted submarine warfare was the use of submarines to sink other ships without giving them a warning about their attack.

https://en.wikipedia.org/wiki/Unrestricted_submarine_warfare

((Yes, I am quite aware that at times Wikipedia is not all that trustworthy but this was an excellent definition of unrestricted submarine warfare. If you want to read about the usage of unrestricted submarine warfare use the following link from a good source.)

Unrestricted Submarine Warfare