When you're intolerant, you can't or won't put up with something. Usually, that means you're not very open-minded.
Being intolerant can mean you have a medical condition, like lactose intolerant people who can't eat dairy products. But more often being intolerant means you're unwilling to tolerate other ideas, opinions, beliefs or cultures. Homophobic people are intolerant of gay people. Sexist people are intolerant of women. Racist people are intolerant of people from other races. So it's safe to say intolerant people aren't the friendliest folks.