Synchronised. You probably hear that word a lot. However, be careful, because it can mean different things depending on the context.
First, "synchronized" is a Java keyword, which relates to "synchronized blocks." This is what online tutorials will tell you about and it is mostly what synchronized means in Java. The synchronised keyword is a means of achieving mutual exclusion, that is, preventing concurrent access to shared data.
Second, "synchronised" can refer to a "main thread." In this regard, "synchronise to the main thread" and "execute a synchronised task" both mean "execute a task on the main thread".
Third, "sync" means "synchronised" - sync is just an abbreviation of synchronised. However, in casual Bukkit speak, "sync" also means "on the main thread." To run something "sync" means to run it on the main thread, to run something "async" means off the main thread.