The file extension FNT denotes a Windows Font File, that is a Windows font files. Fnt files can contain either a bitmap or a vector font. Fnt is a pretty old format, which was already used in Windows 3.0. Meanwhile, TrueType and OpenType fonts have replaced the FNT format.
Fonts are in short all alphanumeric characters, special characters and partly international characters represented in a specific style and cut. Well-known fonts are for example Arial, Helvetica or Times New Roman.
FNT files can still be opened with some programs, but already Windows XP did not register a file type for FNT anymore.