How Wingdings actually encodes its symbols
Wingdings is a "symbol" font: instead of drawing letters, every printable ASCII character from space through tilde (0x20 to 0x7E, 95 characters total) is assigned a small picture instead, such as an envelope, a pair of scissors, a zodiac sign, or a hand pointing. Type a capital "J" in Wingdings and a smiling face appears; type a lowercase "j" and something else entirely shows up, since the mapping runs off the raw character code, not the letter itself.
Windows exposes that same glyph set a second way. Microsoft's OpenType specification, in its section on non-standard (symbol) fonts, states that the cmap table for a font like Wingdings should use Unicode's Private Use Area starting at 0xF000, adding that "it is suggested to derive the format 4 encodings by simply adding 0xF000 to the format 0 (Macintosh) encodings." In practice that means the smiley assigned to ASCII "J" (0x4A) also lives at Unicode code point 0xF04A, a second address for the identical glyph.
Two different outputs, and why this tool defaults to the one that always renders
The "Windows font code" mode above reproduces that exact 0xF000 offset. It is the real Wingdings trick, and on a Windows machine with the Wingdings font installed (most of them, since Windows ships it by default), the output displays as proper symbols. Copy it into Word or a Windows-rendered web form and the pictures show up correctly. On a Mac, a phone, or a Linux machine without that font, the same text shows up as empty boxes instead, because there's no glyph to draw.
"Symbol preview" mode, the default, sidesteps that problem by mapping each character to its own separate Unicode symbol instead of a Wingdings-specific code point: real, independently encoded characters from blocks like Ornamental Dingbats and Geometric Shapes Extended, several of which Unicode added specifically to give older symbol fonts a portable equivalent. This is plain Unicode text, not a font file or an image, so it renders the same everywhere without requiring anything installed on the visitor's device. It's a close standalone equivalent to each Wingdings glyph, not a pixel-for-pixel reproduction of the original artwork.
Decoding text back into words
The third mode runs the font-code conversion in reverse: paste in text that used the 0xF000 offset (copied from this tool's own font-code output, or from anywhere else that encoded a string the same way) and it subtracts 0xF000 back off each character to recover the original letters. This works even on a device that can't display the symbols in the first place, since decoding only needs the underlying character codes, not a rendered picture.