Fira Code is an extension of the Fira Mono font containing a set of ligatures for common programming multi-character combinations. This is just a font rendering feature: underlying code remains ASCII-compatible. This helps to read and understand code faster. For some frequent sequences like .. or //, the ligatures allow one to correct spacing.

This font is expected to work in most text editors but won't work in most (especially VTE-based) terminal emulators. A detailed list is available on https://github.com/tonsky/FiraCode#terminal-support