Surtitles are like subtitles, but for live shows where people are speaking a different language. When you go to a show, you can see the surtitles projected above the stage, usually on a screen or on a piece of fabric, and they translate what the actors are saying into a language you understand.
For example, if you go to a theater and the actors are speaking in French but you only understand English, the surtitles would show up above the stage and say things like "Hello, my name is Jacques" or "I am going to the store." This helps you to follow the story and understand what the characters are saying, even if you don't speak the same language.
The surtitles usually appear in the same font and color as the subtitles you see on TV, and they are synchronized with the actors' lines so you can read them at the same time as the characters are speaking. This helps you to feel like you are part of the show, even if you don't understand the language.
Overall, surtitles are a great way to enjoy live shows in other languages, and they make it easy for people from different cultures to come together and enjoy the same performance.