If you mean the sometimes-stated idea that white people cannot be subject to racism (because it requires prejudice + power), I generally don't agree.
From memory, the idea originated from a paper where the author was talking specifically about institutional racism and so, when defining racism for the specific context of her paper, she said that (in her paper), "racism" meant institutional racism. My impression (and it has been years since I looked at this) is that some people ran then with the idea that it was the only definition of racism (which it isn't, and nor was it ever intended to be).
The "prejudice + power" model is probably very useful when analyzing institutional forms of discrimination, and in white majority countries that would generally mean that white people are seldom the victims of institutional racism (though I would guess that, for example, a particular employer could be institutionally racist against white people, despite a wider societal bias in favor of white people) but it isn't the only form of racism - there's plain old, day-to-day, interpersonal racism too.
Not a topic I fancy delving deeply into, nor is it the topic of the thread or one where I have any real expertise, but since you asked, the above is my general impression.