This is antique Xonix game. Last time I saw it 8 years ago on PDP-11.The rules are apparent.
Your can capture square rounding it by your "tail" and the aim is to capture more then 60% of field square.
You must avoid red "Xonixs" cut you "tail".
Control keys:
move left;
move right;
move up;
move down;
Space stop;
move left;
move right;
move up;
move down;
Space stop;