Strength is the property of being physically strong (you can do, say, 100 push-ups) or mentally strong (you can calculate percentages in your head while people are shouting at you).
Strength has several shades of meaning. The strength of something can be the measure of how much force or pressure it can withstand over time (like a flood wall). Or its potency (like a pesticide). Or its intensity level (like a radio signal). Its fighting capabilities (like an army). Or its specialty: A chameleon's strength is actually in its ability to blend in with its surroundings.