String array to 2D char array

Hello, I don't really understand how the 2D array works here. Why can't we take a normal array? Can someone explain this? Is there any other way to convert String array to char array? Thank you!

String[] arr = { "apple", "banana" };
char[][] cs= new char[arr.length][];

for (int i = 0; i < arr.length; i++) {

	String string= arr[i];

	for (int j = 0; j < string.length(); j++) {

		cs[i] = string.toCharArray();
	}
}