Learn Spanish (Latin America): Son las cuatro y los jinetes rompen a galopar por el hipódromo. - It is four o'clock and the jockeys break into a gallop around the racetrack.

The phrase romper a followed by an infinitive (like galopar) is used to describe an action that starts very suddenly and with intensity. While empiezan a galopar just means they start to gallop, rompen a galopar gives the vivid, sudden image of the horses breaking into a gallop right as the clock strikes four.

Could I use se echan a galopar here instead of rompen a galopar?

Yes, echarse a followed by an infinitive is another way to express a sudden start to an action, especially one involving physical movement. Se echan a galopar would work perfectly well here. Note that romper a is very commonly used for sudden weather (like romper a llover) or emotional bursts (like romper a llorar), while echarse a is great for sudden physical departures (like echarse a correr).

Why do we use por el hipódromo instead of en el hipódromo?

We use por here to express movement through, around, or along a physical space. If you said en el hipódromo, it would merely state their static location, meaning the galloping is happening at the racetrack. Por captures the dynamic action of the jockeys moving around the track itself.

Does specifically mean jockey?