。
She tucked it away as a precious keepsake.
Literal
She [topic-は] that [object-を] [treasuringly-後生大事に] [kept-stored-とっておいた].
後生大事 (goshō-daiji) is a four-character compound with Buddhist roots: 後生 ('the next life, the afterlife') + 大事 ('important, precious'), giving 'as important as one's afterlife' — i.e., 'utterly precious, treasured beyond measure.' The に converts it to an adverbial: 後生大事に ('treasuringly, with utmost care'). 取っておく ('keep, set aside') is 取る ('take, hold') plus the auxiliary ~ておく ('do in advance / leave in a state'), giving 'tuck away, save up, hold on to.' Together: 'kept it tucked away as if it were precious beyond words.' Slightly ironic in modern usage, often hinting that the speaker thinks the value is exaggerated.