System.Threading.Thread.Sleep(2000);

That statement suspends the current thread for 2000 milliseconds i.e. 2 seconds.

In a single threaded application, you could use this to create a delay which was apparent to the user of the application i.e. to slow it down so intermediate steps would be visible.

In a multi-threaded application, you might want to suspend one thread so that all the processing time was allocated to other threads executing within the application. Otherwise the scheduler would allocate a time slice to each thread.

However, it's not usually a good idea to call Thread.Sleep on the main thread in a GUI application as it will make the user interface unresponsive for the number of milliseconds specified. There are generally better solutions such as asynchronous processing.

Notice also that Thread.Sleep is not reliable for very short delays (about 15 millseconds or less) as its resolution depends on the system clock.