The root 'metric' refers to length, so that makes an isometric muscle contraction one that occurs at a constant length. 'Tonic' in this case refers to the tension generated, so isotonic contraction involves a constant tension (or force), but the length changes. I think it makes the most sense when you imagine picking up a cup of coffee from a table. When you first grab on and try to lift, you don't automatically use every bit of strength your muscle is capable of to shorten your bicep and flex at the elbow. If you did that, every light little object you tried to lift would go careening over your head. Instead, your arm will start off with only a few muscle groups. If that's not enough to counter the weight of the cup it will recruit more and more muscle(building force), until the amount of force your arm is generating is equal to the weight of the cup. This is the isometric phase of muscle contraction- you haven't acually moved anything yet, just built up force. The minute you've generated enough force to just equal the weight of the cup, your muscle begins isotonically contracting. Your muscle gets shorter and shorter, bringing your arm up and the cup along with it.