What does the 'can't find main string' error in Java mean?

The 'can't find main string' error (often `ClassNotFoundException` or `NoClassDefFoundError`) indicates that the Java Virtual Machine (JVM) cannot locate the class containing your `main` method. The `main` method is the entry point of your Java application, and it's essential for the program to start executing. This usually happens because the class containing the `main` method isn't in the classpath or hasn't been correctly packaged and deployed.