It's a common phrase used in France and other French-speaking countries for playing a game of "eleven", which is a variation of hide-and-seek.
Here's how it works:
1. One person is "it" and counts to eleven while the others hide.
2. Once the counter reaches eleven, they yell "Je dix Onze!" and start searching for the others.
3. The first person found becomes the new "it".
So, "Je dix Onze" is essentially the starting point for the game.