The main-safe code is the code that is asynchronous (e.g. fetching the data from the internet) and can be called from the MainActivity/main thread without any issues.
Libraries like Room and Retrofit offer main-safety out of the box when using Coroutines, so you don't need to manage threads to make network or database calls. This can often lead to substantially simpler code.
However, blocking code like sorting a list or reading from a file still requires explicit code to create main-safety, even when using coroutines.