。
She is carrying a backpack on her back.
Literal
She [topic-は] back [on-に] backpack [object-を] is-carrying.
リュックサック is a katakana loanword — but unusually, from German (Rucksack), not English. This makes it a comparative rarity in modern Japanese, where most loanwords come from English. Often shortened to リュック in casual speech. 背おう (背負う) is the verb for carrying something on the back.