Hola Jen

Solía +verb in the imperfect and Imperfect of a verb both mean "used to do something" as we are saying in the lesson. So here: 

Yo solía cantar en un grupo de rock.

I used to play in a rock band. 

you could also say "Yo cantaba en un grupo de rock", and this would mean the same.

However there are cases where Solía + imperfect cannot be used the same way we use the Imperfect of a verb. See this example:

"Los niños jugaban en el parque mientras los padres charlaban"

The children played in the park while the parents chatted

In this case we arebusing the imperfect tense to describe a situation in the past. This is not expressing the same as "solía + imperfect" as here we are not talking about repeated actions in the past. 

We also use the Imperfect tense of a verb (not solía) when we express what "was happening" as an equivalent to "estaba + gerund". For example:

"Ella caminaba por el parque con su perro" (She was walking along the park with her dog) 

""Ella estaba caminando por el parque con su perro" (She was walking along the park with her dog) 

Here you wouldn't use "solía" as it is not talking about a repeated action in the past.

Hope this helps

Saludos

Inma