Well, modifiers is a really broad term. I'm assuming you mean access modifier. If so, modifiers are just a description of what things can access that data.. There's a bunch of them, but the key ones you need to know, from what I know, are:
public: Pretty obvious. All classes can access this piece of data.
EX: public int x;
private: Only things within the class can access this piece of data.
EX: private int x;
protected: Only things in the class and subclass (You know inheritance, right?) can access it. (BTW, this is VERY rarely used from what I know).
protected int x;
Those are for access. There are two other types of modifers that are important that describe a data's abilities. One is final and one is static.
Final: This is normally used for data, but can be used for classes. Anyways, it makes it so that whatever this code is can't be changed. Forever and ever and ever. So, you would want to use it when you want something uchangable, like:
final double pi=3.14;
(Pi never changes, so you want it final)
Static: This means that it can't be used by objects. This allows your code to be run without objects, because as you hopefully know Java is an OOP. So, if I want my code to work without objects, make it static. Like the famous:
public static void main(String[] args)
(This is the common method that is used at the start up of a program. The array in the parameters is used for when you need to pass an array into the main method, mainly used if you make a program in notepad and want to run into in command prompt. But who does that? :P)
Hope this didn't confuse you. I'll happily explain anything in detail, feel free to ask. There's a lot of information I left out, but this is just the bare basics.
