Short links (bit.ly, tinyurl and others) hide where they really lead. With Follow short links turned on in Settings, 1krBlocker asks the short-link service where a link goes, without opening the page, and checks the real address.
The risk: See where links like bit.ly lead, without opening them. This contacts the link service, which may tell the sender that your number is active. Off by default. Some scam short links are made for one victim, so asking the service can tell the scammer the message was seen.
That's why it's off by default. Without it, the link preview still warns "Uses a short link that hides where it really goes (…)", and you can decide not to open it.
If you want to know where one particular link goes without turning this on, don't open it: ask the sender through another channel, or go to the company's website yourself.