It is more than one task running at the same time.
To keep the user interface responsive.
If we don't use concurrency, we'll lower the quality of user experience. such as, When downloading and scrolling images, these actions slow down the app.
By structuring your app so some tasks can safely run at the same time.
Grand Cetral Dispatch, for simple tasks.
First in, First out. such as a waiting line to ride a roller coaster.
It is a useful tool for complex tasks to specify input, output, and helper methods. Additionally, the act of canceling.
The Dispatch Framework was created by Apple as the concept of Grand Central Dispatch. Dispatch is Execute code concurrently on multicore hardware by submitting work to dispatch queues managed by the system.
Concurrent VS serial
Synchronous VS Asychronous
There are DispatchQueue.main and DipatchQueue.global(qos: ...) in GCD Queues. The Main queue is used so often for user interface updates. and this is serial. The other global queues are concurrent they can have more than one tread. qos means quality of service, which has a total of six types.