- refuses to debate or discuss its merits and values
- refuses to meaningfully consider or discuss alternatives
- responds to discussion with aggression
The book which includes his quote was published in 1945, Popper was of Jewish ancestry, and the book was speaking primarily about avoiding totalitarianism. His philosophy should be taken in that context. People are manipulating his quote to try to justify intolerance of anything except their own world view, but it is this exact sort of totalitarianism he was suggesting that may imperil an open and free society. In particular he also defined an open society as one "in which individuals are confronted with personal decisions" as opposed to a "magical or tribal or collectivist society."
Intolerance trends towards a closed society where you believe what you are supposed to believe, or face the consequences. Tolerance trends towards an open society where individuals may not agree, but are free to express themselves and challenge one another on any view or value.