What is "oystron"

Oystron is an action game developed for the Atari 2600 by Piero Cavina and released in 1997. It is one of the earliest homebrew games for the console. The game, Cavina's first, was initially made available as a freeware 4 KB binary file designed for use on the Starpath Supercharger and with Atari 2600 emulators such as Stella and PCAE. It was later released in cartridge form by XYPE, an independent group of Atari 2600 homebrew developers.