One of Sun Tzu's 9 Principles of War, derived from his famous text The Art of War, which states that you should use only as much force as is required to accomplish your objective, while obeying the other principles. His principle of mass seems to contradict this, but it's simple to explain the difference:
If a small squad of troops are advancing on your position, and you have a division (about a thousand troops) in place, don't call for reinforcements -- you already have superior mass. You may only want to send double or triple the number of attackers out to counter the threat, and let everyone else hold the fort.
This rule appears to not apply in real-time strategy games, but there is usually some level at which N tanks can kill an infantry unit and take no damage. This is the ideal balance of mass and economy of force.
When trying to memorize the nine principles using mnemonics, this is almost always the last one, and forms its own mnemonic, EOF (end of file), which tells you that you've successfully remembered them all.