What does ''trod the boards'' mean?

To tread or walk the boards is an idiomatic way of saying "to be an actor." "Trod" is the past tense of the verb "tread." Stages are usually made of wood planks, and actors who perform in live drama must walk on those boards. The usage goes back to the late 1600s, when people would say "tread the stage" as a term for being an actor. In the mid-1800s, people started saying "tread the boards" instead.
In this line from Coraline, author Neil Gaiman has Coraline's quirky downstairs neighbors, Miss Spink and Miss Forcible, describe themselves. They live in an apartment below Coraline's with several old Highland terriers. They tell Coraline that they were both famous actresses in their younger days. Then they say, "We trod the boards, luvvy." Even if you didn't know the meaning of the idiom, what they've already said would explain it. Now the two women are old and round, but they want to make sure Coraline understands that they were important people in their prime, so the first thing Miss Spink says to her is that they used to be actresses.
https://www.dictionary.com/browse/tread--the--boards