Learn Spanish (Latin America): Los habitantes solían reunirse en esta plaza. - The inhabitants used to meet in this square.

The verb soler means to usually do something. When talking about the past, it is almost exclusively used in the imperfect tense (solían) because it describes an ongoing habit or routine, rather than an isolated event with a clear start and end. It fits perfectly alongside past-duration expressions you are learning, as both describe ongoing past situations.

Why does reunir have a se at the end here?

By itself, reunir means to gather or to collect (like collecting money or gathering items). To say that people gather themselves together, which is how we say to meet up in Spanish, you must use the reflexive form reunirse.

Can I put the se before the conjugated verb instead?

Yes! When a conjugated verb is followed by an infinitive, you have two choices for pronoun placement. You can attach it to the end of the infinitive (solían reunirse) or place it right before the conjugated verb (se solían reunir). Both are completely correct and mean the exact same thing.