What it is: Proportionality is a principle of international law. In war, this principle requires that force should not exceed what is thought to be necessary for self-defence, especially where there is a risk of 'collateral damage': the killing of civilians. The proportionality principle is an important part of arguments about just war.
What it means: In practice it means doing the impossible: weighing up the desired 'good' outcome of a military strategy against its destructive methods. In the Gulf War of 1991, when Iraqi forces had invaded Kuwait, some allied forces wanted to occupy Iraqi territory and overthrow its leader, but others thought that the proportionality principle would not allow more than driving Iraqi forces out of Kuwait. If civilians shelter in a military target and are killed by a direct hit on it, according to the proportionality principle that is not unlawful. On the other hand, intentional and, worse, indiscriminate bombardments of civilian areas are disproportionate and a war crime under international law. Arguments about 'legitimate military targets' arise in every war and are seldom resolved. They usually occur after the target, legitimate or not, has been hit and the killing done. The proportionality principle began as a way of curbing the violence of war, but it is difficult to apply and isn't at the top of the 'Don't' list for military tacticians. It also means that the fate of non-combatant civilians - 'ordinary people' - in war continues to be merely part of war's deadly game of chess.
Think about it: The proportionality principle can also be thought about together with what is called 'asymmetric warfare'. 'War on terror' is asymmetric: the might of the military focused on subduing a relatively tiny number of criminal assassins (whose weapons, though they can be deadly, are much simpler and fewer). The conflict in Israel has also been called asymmetric: a state army attacking a people who have no army and relatively few weapons. The proportionality principle is, of course, another example of the problem of 'ends versus means': whether or not a satisfactory outcome can justify the use of inhumane or criminal tactics to get there. Too many people think that it can, forgetting that any good done is tarnished by the wrongfulness of the method. There are always good - and better! - ways of getting things done, but too many people don't take the time or have the patience to work them out. Look at one (or more) of the conflicts you know something about, and consider how it might be handled without invoking the proportionality principle - and with a chance of achieving conflict resolution in humane and non-violent ways.