About synchronized Java (Android) arraylist

Java (Android) is changing arraylist data from multiple threads

At the same time, data was added (.add)

So I learned about synchronized while learning about synchronized

synchronized(data) { data.add(""); }

So I changed it as above But if the phenomenon of adding data at the same time occurs

Does synchronization mean that you wait while the other thread changes?

Or I wonder if you don't access it at all and don't add data.

It's late, but happy new year!~^{^}