Explain some code.

new FirstJoin(this); is calling the constructor which is taking an instance of what you put in and storing it in 'plugin' and its registering the event of itself.

 

Just learn java - it really helps.
Do not get me wrong, sometimes understanding a program/algorithm can be hard, especially when you are dealing with pointers. But if you do not know how a constructor call looks, you should learn java's syntax.

 

This is your constructor, you are passing it an instance of the plugins main class (which extends JavaPlugin) and when a new 'FirstJoin' object is created, you register your events.

 

and apparently classes.
But if you cannot understand how arrays work..

 

When I first learned PHP is took me some time to understand how arrays worked and how they could possibly be useful. I have to say, I can understand how he might not understand them. That said, constructors are basic Java.

 

Not knowing what a constructor is means you cant write anything at all

Find a good guide to java basics. Not some shitty video that tells you what to copy, a real written guide that explains what everything is.

Code (Java):

public FirstJoin(ServerInfo pl)

Constructors are methods that are used to create a new object of a class, they have the same name as the class and no return type. In this case, this is a constructor to make a FirstJoin object, and it looks like it has to be passed the main class of the plugin 'ServerInfo' (which is what new FirstJoin(this) does. 'this' is a variable that points to whatever class object it is used inside).

 

Many scattered answers, lemme break it down for you, and in return, I want you to go read a tutorial on OOP.

Code (Text):

private ServerInfo plugin;

private Class variable;

Code (Text):

  public FirstJoin(ServerInfo pl) {
     plugin = pl;

     Bukkit.getServer().getPluginManager().registerEvents(this, plugin);

   }

/* Constructor
public Class(Class variable) {
}
*/

previously defined variable = anotherVariable;

Class.getInstance().getAnotherInstance().registerSomething(instance, instance);

Code (Text):

public boolean onCommand(CommandSender sender, Command cmd, String label, String[] args) a

public boolean variable(Class variable, Class variable, DataType variable, ClassArray variable)

Code (Text):

new FirstJoin(this);

// 'new' keyword
new Constructor(instance);

This got really messy, didn't expect that. OP, hopefully this explains some of what you want.
https://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html
https://docs.oracle.com/javase/tutorial/java/concepts/class.html
https://docs.oracle.com/javase/tutorial/java/nutsandbolts/variables.html
https://docs.oracle.com/javase/tutorial/java/javaOO/constructors.html
https://stackoverflow.com/questions/5126082/what-exactly-is-an-instance-in-java
https://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html

Some OOP tutorials: https://docs.oracle.com/javase/tutorial/java/concepts/
http://www.aonaware.com/oop1.htm
https://www.ntu.edu.sg/home/ehchua/programming/java/J3a_OOPBasics.html