Computer code is a series of executed statements. Frequently, these statements are executed one at a time. If one part of your code takes a long time to run, the rest of your code won't run until that part is finished.
However, this isn't how it has to be. We can often make the exact same code go much faster through parallelization, which is simply running different parts of the computer code simaltaneously.
The first example of this is asynchronous code. The idea here is that many times you do things like send a call to another computer, perhaps over the internet, using an API. Normally, code then has to simply wait for the other computer to give it a response over the API. But asynchronous code can simply keep on going and then the API call returns later.
This makes code harder to reason about and handle because you don't know when the API call will return or what your code will be like when it returns, but it makes your code faster because you don't have to wait arou