As always, it would help to look at the uncontracted form:
It is people who cannot understand me!
"It" is a dummy pronoun, as "people who can't understand me" (*) would be ungrammatical with the "who".
Alternative ways of phrasing this idea are:
- The/My problem is that other people can't understand me!
- It's other people's fault that they don't understand me!
- It's other people who can't understand me!
This sentence sounds very natural when used in the right context because the subject, other people or people in general, is placed at the very front of the sentence. You could say that this is an example of the topic-comment structure used in English.
From the above, it follows that "these are the people who don't understand me" has an entirely different meaning.