Abbie's two-line poem plays on the phrase "mother tongue," which usually means the first language we learn at home. Here, that language is silence. The speaker grew up surrounded by it, so they can speak it naturally, almost without effort. But the second line admits a painful gap: being fluent doesn't mean understanding.
That contradiction is the heart of the poem. Silence is something the speaker knows how to do, perhaps how to keep, but they can't decode what it means. The poem suggests a home or relationship where things went unsaid, and the speaker picked up the habit without ever learning the reasons behind it. Judging by the wording, the silence may have come from a mother, though the poem doesn't say.
The mood is quiet, wistful, and a little lonely. There's confusion in it, and maybe a longing to be able to finally hear what the silence was trying to say.