Asynchronous

When something is asynchronous it just means that something can happen at a later point in time and instead of waiting for it, other things can be done in the meantime.

In software engineering it means the same, but it is applied to code. When a function or block of code is asynchronous it is simply saying that we do not know when this asynchronous action will be completed so the rest of the code can continue being processed whilst we wait for it so that it does not block other actions.