A "Field Expedient", in my experience, is a jerry-rigged or makeshift substitute, frequently/usually made from on-hand components on the spur of the moment, replacing an item that you WANT to have, but don't, and you need. It's a substitute born of necessity created with the materials at hand.
The "Field Expedient" icon would probably be the TV character 'McGiver'. A simple example of a Field Expedient would be a shoelace and stick used to fabricate a makeshift tourniquet, or a couple of belts, some sticks and somebody's jacket used to make an emergency stretcher. Another good, if tragic, example is McVey's use of fuel oil and fertilizer to make one hell of a bomb!
How it affects me? It broadens my ability to meet situations and solve problems with the materials at hand and enables me to pack less stuff, by packing MORE stuff that can be multi-tasked.