Intolerant is a word that means that a person does not tolerate something.
It is usually used to mean that a person is nasty to anyone with ideas that they don't agree with (i.e. the opposite of tolerance). It sometimes actually means that the speaker of the word does not like the person described as intolerant.
Amusingly, conservatives are the ones who have the reputation for being intolerant. This is certainly true in some cases. That being said, I'd argue that people from the left side of the spectrum can be intolerant too.
In my view, when a person uses the word intolerant to describe someone they usually mean "intolerant of ideas I like." In contrast, I'd argue that every political viewpoint considers some ideas to be beyond the pale. Thus, I think you'll find that some people on the left are rather intolerant of racism and sexism and the like.
Mind you, I'm inclined to think they are correct on that, but I think you'll also find some people on the left to be rather intolerant of any opposing view--and the same is true of more conservative people on the right side of the political spectrum.
See John Calvin, Rush Limbaugh, ultra conservative, conservative, Christian Reformed Doctrine, or Christian Reformed Church for possible examples of the above. You can also see liberals or political correctness.