If you’re not familiar with it, Samba is an open-source implementation of the SMB/CIFS networking protocol. It allows many Unix-based operating systems to connect to Windows file shares and printers, making it easy to transfer files over a network between operating systems. Out of nowhere, Google has released a Samba client for Android.
The app is pretty barebones, but it gets the job done. Just enter the file share you want to connect to, plus a username and password if required. Once you tap Mount, the share should instantly appear in the Downloads/Files app, allowing you to easily copy files back and forth. Opening files from the share works too – I was able to play a video file straight from my home server (granted, it was a low-resolution WMV file).
The application is also open-source, and you can find the code for it on GitHub. If you’re looking for a more powerful file manager with Samba support, I recommend Solid File Explorer, but this app is great for basic operations. Google should really bake this into the Files app on Android O.