For clarity, the surrounding context of that line reads as follows:
The cover goes first, then outer leaves
curling away, then spine and a scattering.
Truth, brittle and faint, burns easily,
its fire as hot as the fire lies make—
flame doesn’t care.
(Bold added for easy reference of line 6.)
The speaker examines the way a fire devours a book. The pages are personified as they huddle together in the midst of the flames, trying desperately to protect each other from being obliterated by the fire. But in the end, the fire is victorious. After eating the cover and then the outer edges of the pages, it burns both the truth and lies. So this line is significant in two different ways.
First, fire (and therefore attempts at censorship) does not discriminate between the truth and lies. Regardless of the author's intent and regardless of a book's potential for good or evil purposes, fire consumes both without the burden of concern. Therefore, both truth and lies are equally at the mercy of those who choose to burn books.
Second, the speaker uses a simile to convey the power of lies—that they create hot fires themselves. This reaches beyond the physical temperature of the fire as the reader imagines what ideas have landed these books in the fires to begin with. Whatever truth and whatever lies exist in those pages, they have ignited a fiery passion within people who wish to see the words destroyed.
It is also worth noting here that the books are not completely destroyed, as the speaker notes that a few words can be found in the ashes. This also conveys the longevity of the ideas behind literature—the power of words to survive is more powerful than those who seek to completely destroy them.