FlashAir cards are really nifty SD cards that allow you to combine SD card storage on your printer with the convenience and speed of uploading to the SD card via WiFi. Using WebDAV (make sure you only buy FlashAir W-04 or W-03 cards - W-04 are the newest and most robust), you can mount the FlashAir over WiFi as a drive on your desktop/laptop/server/SBC and drop gcode/x3g files directly onto the card.
Heads-up
Not all firmwares out there support FlashAir cards. Check with your firmware vendor before buying.
It can take some effort to properly configure the cards, but once they are set up properly, they tend to be rock solid.
First, make sure you use the configuration app that Toshiba provides (it can be buggy and doesn't have great documentation, so beware) to initialize the card and turn on WebDAV. Next, build a configuration file using the information for the CONFIG file located in the FlashAir Developers website API section. Be sure you turn on station mode and NOT pass though mode or AP mode. If it isn't clear how to do so from the API documentation, there is also a tutorial in the Tutorials section of that same site.
Don't use noise= settings, as they reduce the range of your card, and your printer is most likely made of lots of metal and stepper motor drivers tend to create a lot of EMI that you're already going to have to overcome, without the signal dampening effects of all that metal around your card.
Whatever you do, do NOT touch the CID line in the config file. If you do, you will need to reinitialize the card using the configuration app to get it restored before the card will work, and it may take multiple attempts to do so (not that I've done that by accident or anything...).