From their GitHub repo page:
FontForge is a free (libre) font editor for Windows, Mac OS X and GNU+Linux. Use it to create, edit and convert fonts in OpenType, TrueType, UFO, CID-keyed, Multiple Master, and many other formats.
FontForge also has a book about how to design typefaces through its software (hosted on GitHub Pages, no HTTPS connection).
Nice app, although Windows has its own font editor by default, eudcedit.exe, but FontForge is a great alternative for those who often require creating fonts (designers).