Apple Lossless Audio Codec (ALAC), often known as simply Apple Lossless, is Apple's (formerly) proprietary lossless audio codec. It is equivalent to FLAC, and can play on most of Apple's devices. The file is stored with a .m4a file name. Since iTunes and iPods cannot play FLAC without an alternative OS like Rockbox, ALAC is a desirable audio codec. But, ALAC files tend to be slightly larger than FLAC.
To convert FLAC or other formats to ALAC, you can use an application such as XLD (for macOS) or Foobar2000 (for Windows).