。
She was climbing up the ladder.
Literal
She [topic-は] ladder [object-を] was-climbing.
~ていた is the past form of ~ている, expressing a past ongoing action: 'was climbing.' With activity verbs like 昇る ('climb'), it gives the standard past progressive. The subtle contrast with 昇った ('climbed,' a completed action) is the same as English 'was climbing' vs 'climbed.' Where Japanese diverges from English is with change-of-state verbs — there ~ている describes the resulting state ('is dead,' 死んでいる) rather than the action in progress.