That is manually putting the board into bootloader mode to force an upload, typically only needed to be done if an interrupted/bad upload occurred and the board is not responsive.
Arduino does that on its own when you click the upload button.
I wrote this a while back, so I’m just going to paste it here:
Proffieboard use 2 different drivers on the computer, depending on how it’s booted.
Just like a PC running Windows would boot into normal Windows OS or into the BIOS to set core settings, or an Android phone can boot into normal Android or bootloader,
Proffieboards either boot into normal mode running ProffieOS, or bootloader mode.
Each of those states looks like a different device to the PC. Different devices require different drivers to talk to your computer properly.
If you install the wrong driver for a device, it’s not going to work, simply put.
When booted normally and running ProffieOS, the board show up as a “Proffieboard”, a composite serial device (CDC data etc…).
Windows typically sees this device when first connected, installs the driver on its own, and will use usbser.sys driver.
The board should be seen in on a COM port in Arduino under menu Tools>Port.
It will not show up here if in bootloader mode.
When it’s in bootloader mode and showing as STM32BOOTLOADER, it needs WinUSB v6.1.7600.16385 driver installed with proffie-dfu-setup or Zadig.
It’s a one-time setup installation when the PC sees the board for the first time in bootloader mode.
It needs this because Arduino is going to put the board in this bootloader mode when you hit Upload so that the board can receive programming.
Then when the upload is complete, the board reboots back into normal ProffieOS.
*** If you do not put the Proffieboard into bootloader mode before running Zadig, you are doing it wrong, and installing the wrong driver for the board because it’s still connected as a “Proffieboard” composite device at that point.
Putting the board into bootloader mode so you can run Zadig and install the correct driver for STM32BOOTLOADER device can be done 2 ways, either with the onboard buttons, or via Serial Monitor in Arduino.
Button sequence is HOLD BOOT, PRESS and RELEASE RESET, then RELEASE BOOT.
Alternately, you could also hold BOOT with one hand, and connect the other end of the USB to the computer while holding BOOT.
Either result in the same sequence that is required; powering on the board while the BOOT button is held.
Serial Monitor command is simply choosing the COM port with the “Proffieboard” under Arduino menu Tools>Port, and sending the command RebootDFU.
If the computer has forgotten the driver, or the driver was overwritten, or SOME stupid Windows thing happened… you might just need to re-run Zadig.
If things still don’t continue working once this is done, you may have another device using a conflicting driver, see this thread for info: