What is meant by the last line, "Its silence silences," in the poem "A Photograph" by Shirley Toulson?

What she means by this is that in the final analysis, there's nothing more to be said about the old photograph of the speaker's mother and the memories it conjures up. The photo speaks for itself; it shows rather than tells. As a picture, as a visual representation of reality, it cannot adequately be articulated by words, no matter how eloquent they may be.
There comes a point when mere language is no longer enough, and this point has been reached by the end of the poem. All that's left is the silence of the photograph—an eloquent silence, to be sure, but silence all the same. But silence here is not just the absence of sound; it is, rather, a force in its own right that actively silences the words that have come before it, rendering them almost—but not quite—obsolete.
http://www.english-for-students.com/A-Photograph.html