Serial/parallel garbage collectors are stop-the-world events, meaning that they will stop all running application threads and perform the GC. Once this is done, collectors resume the application threads. While the GC runs, the application pauses for that time.
The difference between serial and parallel garbage collectors is that serial garbage collectors are single-threaded and parallel ones are multithreaded. When these collectors are running, the application will stop for that time.
Serial and parallel garbage collectors pause the application while they run GC, and the parallel collector is the default collector. These collectors work better for smaller heaps as it is quicker to clean them and stop the application while reclaiming the memory. With a smaller heap, the application pauses as the GC runs, is likely to be shorter, and mostly goes unnoticed.