"String" is a pre-defined
standard class in
Java which has a lot of
member functions for doing things with strings.
String s1 = "abc";
The methods like replace(), tolowerCase() etc. are accessed by using s1.replace(), s1.tolowerCase() etc. When you call each of these methods you have to pass parameters what parameters the method requires you to pass, like for example if the method requires you to pass an int you have to pass an int even if it's big and it makes your weewee hurt you still have to pass it. An int is an integer and they're very pointy and they hurt.
For e.g. s1.charAt(2) will return the third character in the string s1. (note the index of string starts from 0, i.e. first character has index 0, and so on, starting at zero which is before one and it's a very small number that it's so small you can't see it but it's there at the beginning of every string that you make in the Java language that you program with when you're programming in Java which has strings.)