An exact copy of the original
WOFF, the first web font format, wraps a desktop font and compresses each of its tables with zlib, the compression ZIP files use. Unpacking it reverses exactly that, so the TTF you get back matches the one that was packed in every table, byte for byte, glyph outlines included. We checked this by packing fonts into WOFF and unpacking them again.
That makes WOFF different from WOFF2, which rearranges the glyph data to compress it better. A font unpacked from WOFF2 is equivalent to the original but its glyph table isn't byte-identical; from WOFF it is.
TTF or OTF?
A WOFF records which kind of font it contains, and Refont reads that before converting. If the menu shows TTF, you get a TrueType font. If it shows OTF, the font inside has CFF outlines; choose "TTF (rebuild outlines)" if you need TrueType specifically, as explained on the OTF to TTF page.
Where WOFF files come from
WOFF was the standard web font format from about 2010 until WOFF2 took over, so older webfont kits, theme folders and website backups are full of them, usually next to EOT, SVG and TTF copies of the same font. If a kit already contains a TTF, use that rather than converting.
Webfont kits made by generators of that era were often subset to save bytes, so a font unpacked from one may be missing characters. The glyph count in the report shows how complete it is.
Converting doesn't change the license. A webfont license usually covers serving the font on a website only, not installing it on a computer. Use this for fonts you own or that are openly licensed. How font licensing works