Java Stacktrace error Unsupported major.minor version 51.0

I made this script, and it gives me this error:

java.lang.UnsupportedClassVersionError: net/glitching/client : Unsupported major.minor version 51.0
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClassCond(Unknown Source)
    at java.lang.ClassLoader.defineClass(Unknown Source)

    at java.security.SecureClassLoader.defineClass(Unknown Source)
    at java.net.URLClassLoader.defineClass(Unknown Source)
    at java.net.URLClassLoader.access$000(Unknown Source)
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
Exception in thread "main" 

What does it mean? This is my code...

package net.glitching;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.net.Socket;


public class client {

    static Socket sock = null;

    public static void main(String[] args) {
        while (true) {
            try {
                sock = new Socket("localhost", 4000);
                ObjectInputStream in = new ObjectInputStream(sock.getInputStream());
            } 

            catch (IOException ex) {}
        }
    }

}