Brand - Collective image/representation of one companies product portfolio. With the addition of a company message.

4 key elements of branding

1. Name
2. Logo
3. Slogan
4. Tone

Subway eat fresh

I think they chose this slogan because it's short so it's easy to remember and eating fresh food is healthier so it tells people that subway is healthy. When people are deciding where to eat they'll remember the slogan and know that it's healthy so they'd prefer to go to subway instead of an unhealthy place like mcdonalds.