。
She bought two pairs of socks.
Literal
She [topic-は] socks [object-を] two-pairs bought.
二足 uses the counter 足 ('foot') for pairs of footwear — socks, shoes, sandals all take 足 since they come in pairs and go on feet. The number-counter compound (二足) sits adverbially after the object-marked noun without an additional particle, a typical pattern for floating quantifiers. The reading is にそく — 足 takes the on'yomi reading そく when used as a counter (compare to its kun'yomi あし for the body part).