About FileReader and FlieWriter

i was build my replay system and i find a tutorial in wiki.

but when it comes to the following code, i couldn’t fully understand what it means.
TUniquePtr InfoFileArchive(IFileManager::Get().CreateFileReader(InfoFilename));
if (InfoFileArchive.IsValid() && InfoFileArchive->TotalSize() != 0)
FString JsonString;
InfoFileArchive << JsonString;
Info.bIsValid = true;

hope someone can explain how it works