In computer science, serializing means turn data into a stream.
However, in
English it can also be used to state "one after the other".
You don't want multiple writes to happen at the same time, but you do want them to happen one after the other ==> hence you want to serialize them.
However, that still doesn't answer some questions, their phrasing is not ideal.
To keep things simple, I understand it works as follows:
You can have many readers at the same time.
When there are readers, you can't have writers.
Once there are no more readers, a write can get the lock.
Only one writer can have the lock at any given moment.
This answers all scenarios. Let me know if there's something else I can help you with.