。
She studies very hard.
Literal
She [topic-は] very with-all-might studies.
一生懸命 ('with all one's might') is a four-character idiomatic compound that originally read 一所懸命 ('staking your life on one place,' originally referring to samurai defending their domain) and morphed into the current shorthand for putting all your effort into something. It pairs with effort verbs — 勉強する, 働く, 頑張る — and is one of the most-used set phrases in everyday speech.