That's what they were known as back then

Hello. My name is Eduardo and I’m listening the content “Conversations with an Actor, Part 1”. I’m having some difficult to get the meaning of this phrase: “That’s what they were known as back then.” I need some help here. I’m glad if someone could help me.

known as = what was special about them (in this context).
back then = before.

I hope that helps.

Thank you. It helps a lot. Now it is clear!