Two short stanzas capture a single, still moment: a woman standing with the sun behind her, so that the light shows the outline of her hip through her dress. The speaker doesn't describe her face or say who she is. He only notices the shape of her body in the evening light, and that small detail is enough to hold his attention.
The feeling is one of deep ease. The final line, "so content I hardly speak," suggests happiness so full that words would only get in the way. There's a hint of desire in the glimpse of her hip, but it's gentle rather than urgent, folded into the calm of the countryside.
The mood is peaceful and intimate. Short lines and plain language slow the poem down, much like the evening itself. The title, "Idyll," fits: it names a perfect, untroubled scene, and the poem simply lets the speaker enjoy it in near silence.