。
She has a fair amount of life experience under her belt.
Literal
She [topic-は] considerably life-experience [object-を] has-accumulated.
経験を積む ('accumulate experience') is the standard idiom for gaining experience over time — 積む literally means 'pile up, stack,' giving the phrase a sense of incremental, gradual accumulation, layer by layer. 人生経験 ('life experience') is the broader 経験 narrowed to lessons learned through living, not just professional skill. かなり ('considerably, fairly, quite a bit') is a measured intensifier that signals a meaningful but not extreme amount — softer than すごく but stronger than 少し. The whole phrase often appears in compliments to someone older or in evaluations of someone's wisdom or composure.