Learn Spanish (Spain): En aquella época, la gente vivía en el campo. - In that era, people lived in the country.

Vivía is the imperfect tense of vivir. We use the imperfect here because it describes an ongoing situation or a habit in the past, rather than a single completed event. Phrases like en aquella época (in that era) are strong clues that you need the imperfect to set the scene of how things used to be.

English says "people lived" (plural), so why isn't it vivían?

In English, "people" is plural, but in Spanish, la gente is a singular feminine noun (think of it like "the crowd" or "the population"). Because it is singular, it takes the third-person singular imperfect ending -ía, giving us la gente vivía.

Why use aquella instead of esa for "that"?

Both esa and aquella mean "that", but aquella is used for things that are further away in space or time. Since en aquella época refers to a distant historical era, aquella gives the strong feeling of "way back then" rather than just the recent past.

Does el campo mean "the camp"?

No, el campo is a false friend. It means "the countryside" or "the field". If you want to talk about a campsite where you pitch tents, you would use the word el campamento.