%player%, %server%, and so forth

Code (Text):

#The player in this context
Player player;

#The message in which the variable %player% is defined
String message = "Hello %player%";

#message.replace(String, String) replaces all occurences of the first argument with the second.
message = message.replace("%player%", player.getName());

player.sendMessage(message);

 

For premium resources, when a player downloads a plugin, hardcoded strings like "%%__USER__%%" are replaced with the values they're supposed to be replaced with. These don't work with non-premium plugins or plugins which were not downloaded on spigot