The speaker is giving a warning, almost like a stern piece of advice repeated until it sinks in. Each stanza tells us not to be fooled by a woman's expression, whether she is smiling or frowning, happy or in tears. Both kinds of faces, the poem insists, are lies.
The structure does a lot of the work. The first two stanzas are nearly identical, and then the third strips them down: the smile and frown disappear, along with the words "happy" and "sad," until only "faces are a lie" remains. The poem seems to narrow its claim from particular expressions to every expression. The last two short stanzas turn the warning into something blunt and almost chant-like.
The mood is guarded and distrustful. The speaker sounds like someone who has been burned, though the poem doesn't say how. Judging by the repetition, the warning may be as much for the speaker as for the reader, a reminder not to fall for it again.