。
She does aerobics once a week.
Literal
She [topic-は] week [per-に] once aerobics [object-を] is-doing.
エアロビクス is a direct loanword from English 'aerobics' — typical of Japanese fitness vocabulary, which leans heavily on katakana imports (ヨガ, ピラティス, エクササイズ). やっている uses the casual やる (to do) with ~ている for a habitual ongoing activity, while 週に１回 sets the frequency. The choice of やる over する is slightly more conversational and laid-back, fitting for a casual mention of one's exercise routine.