Windows x86 (32-bit) is a metafamily of graphical operating systems developed and marketed by Microsoft. Using host OS (Windows, Linux 32/64 or Mac OS X) you can build native application for Windows x86 platform.
For more detailed information please visit vendor site.
Examples are listed from Windows x86 development platform repository:
; Latest stable version [env:latest_stable] platform = windows_x86 board = ... ; Custom stable version [env:custom_stable] platform = firstname.lastname@example.org board = ...
[env:upstream_develop] platform = https://github.com/platformio/platform-windows_x86.git board = ...
MinGW - Minimalist GNU for Windows