What does "[backends] should seldom or never need to wait for a write to occur" mean?

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/13/runtime-config-resource.html
Description:

https://www.postgresql.org/docs/13/runtime-config-resource.html#RUNTIME-CONFIG-RESOURCE-BACKGROUND-WRITER

says:

"There is a separate server process called the background writer, whose
function is to issue writes of “dirty” (new or modified) shared buffers. It
writes shared buffers so server processes handling user queries seldom or
never need to wait for a write to occur."

It's not clear what "wait for a write to occur" means: a write() syscall or
an fsync() syscall?