Okay kiddo, have you ever heard of a big show or performance that a lot of people go to see? Well, sometimes the government will help put on these big shows, and that's called "opera publica".
Basically, the government is using some of the money that people pay in taxes to help make the show happen. This is because they think it's important for people to have access to culture and the arts.
It's like when your school puts on a play or concert, but on a much bigger scale. And instead of just your parents and friends coming to see it, lots and lots of people in the whole city can come and watch.
The government's support might mean that they pay for the theater where the show is held, or for the costumes and sets, or for the actors and musicians who perform. It all depends on what the show is and what it needs.
So, opera publica is basically just a way for the government to help make sure everyone can enjoy and appreciate the arts. And who knows, maybe one day you'll get to see one of these big shows yourself!