What does this mean?

I am trying to edit a plugin, but when I drag it into the plugins and start the server I get this error:

19:09:26 [SEVERE] Could not load 'plugins\StonedPvP.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError
: main/mainclass : Unsupported major.minor version 51.0
at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.j
ava:184)
at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.
java:305)
at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager
.java:230)
at org.bukkit.craftbukkit.v1_5_R3.CraftServer.loadPlugins(CraftServer.ja
va:239)
at org.bukkit.craftbukkit.v1_5_R3.CraftServer.<init>(CraftServer.java:21
7)
at net.minecraft.server.v1_5_R3.PlayerList.<init>(PlayerList.java:55)
at net.minecraft.server.v1_5_R3.DedicatedPlayerList.<init>(SourceFile:11
)
at net.minecraft.server.v1_5_R3.DedicatedServer.init(DedicatedServer.jav
a:106)
at net.minecraft.server.v1_5_R3.MinecraftServer.run(MinecraftServer.java
:382)
at net.minecraft.server.v1_5_R3.ThreadServerApplication.run(SourceFile:5
73)
Caused by: java.lang.UnsupportedClassVersionError: main/mainclass : 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 org.bukkit.plugin.java.PluginClassLoader.findClass0(PluginClassLoader
.java:80)
at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.
java:53)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Unknown Source)
at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.j
ava:173)
... 9 more

I have no clue what it means. If anyone knows, please tell me. I am also using bukkit-1.5.2-R01.

 

Here is your answer: <Edit by Moderator: Redacted bit url>

This means that the plugin you are trying to install was compiled with Java 7, but you are running a version below Java 7 (guessing Java 6). Either tell the developer of that plugin to compile it in Java 6 or just simply download and install Java 7.

EDIT by Moderator: merged posts, please use the edit button instead of double posting.