Large folio support is a capability required for a Linux filesystem to fully support large block sizes--block sizes larger than the usual page size, which for ext4 on x86 is 4KB. So now ext4 can support block sizes of, e.g., 64KB--a common maximum block size supported by NVME drives--without having to issue more than a single read/write operation.
Background: recently the Linux memory management architecture completed the transition to so-called "folios", which provide a way to (mostly) atomically manage blocks of memory larger than the normal page size. It's like large pages, but more of a software abstraction than an interface over large page support in hardware. But to leverage the full potential of folios, various components, like the ext4 filesystem implementation, need to be further refactored beyond what was required for the initial transition to the folio API. See https://lwn.net/Articles/1015320/ for a recent overview of folios.
