。
She makes it a habit to exercise before breakfast.
Literal
She [topic-は] breakfast [before-前に] exercise do [decided-ことにしている].
~ことにしている means 'to have decided to do X (as a regular practice)' — it expresses a deliberate, ongoing personal rule or habit. 運動する ('to exercise') is the activity. The pattern distinguishes between externally imposed habits (~ことになっている, 'it has been decided that') and self-imposed ones (~ことにしている).