。
She laid the baby on the bed.
Literal
She [topic-は] baby [object-を] bed [on-に] laid-down.
横たえる (よこたえる) means 'to lay something down,' 'to lay horizontally.' It's the transitive counterpart of 横たわる (to lie down, intransitive). The distinction between transitive and intransitive verb pairs is pervasive in Japanese: 横たえる/横たわる, 起こす/起きる, 寝かせる/寝る — the transitive form acts on someone else, the intransitive describes one's own state.