It was heavily used in the early-to-mid 2000s for storing low-resolution, low-filesize videos on phones, often referred to as "3G video" 1.
I can provide more specific instructions or suggest the best converter for your needs. k.109.3gp
The best option, as it plays almost any file format without needing extra codecs. QuickTime Player: Native player on macOS. It was heavily used in the early-to-mid 2000s