Revivalism in architecture is like playing dress-up with buildings. Think about how you might dress up like a superhero or a princess that you read about in a book – revivalism does the same thing with buildings.
Basically, it means that architects and builders try to make new buildings look like ones from a long time ago, maybe hundreds of years ago. They do it by copying the styles, shapes, and details that were popular in those old days.
For example, you might see a building today that looks like it belongs in a fairy tale from the 1700s, with big stone columns, fancy carvings, and a pointed roof like a castle. This building was probably made recently, but it's designed to look like something from the past. Sometimes this kind of architecture is called "neo-" something, like "neo-gothic" or "neo-classical", which means "new-gothic" or "new-classical".
Revivalism can be a very cool way to honor history and architecture, but it's not always easy to get it right. Architects and builders have to do a lot of research to make sure they're making something that actually looks like it's from the olden days. They also need to use modern materials and construction techniques to make sure the building is safe and sturdy. It's a bit like trying to make a really good costume for Halloween – you need to get creative with what you have, and make sure it won't fall apart!