CircuitPython supports two font formats: the textual .bdf format and the binary .pcf format. By taking the extra step of converting your font to .pcf you make fonts load faster and also typically save some storage space on the board CIRCUITPY flash drive.
The converter software is hosted on github.io. Thanks to technology called emscripten, it runs entirely in your web browser—the font file is not uploaded to a server, which also makes it really quick. web-bdftopcf is derived from the classic font converter of the same name, a program for Unix/Linux systems. If you're interested, you can browse the C source on github.
After a moment, the font will be prepared in .pcf format and depending on your browser settings it may be automatically downloaded or you may have to confirm that you want to download the file.