Note: I'm not 100% certain of my descriptions vs. how others might define things; hopefully this is enough to get an intuative idea of what's going on.
The Strategy pattern lets you change how a function or method does its job. It might be helpful to think of it in terms of refactoring shapes. When you have two functions with similar parts at the top, you break that top part out into a new function. When they have similar parts at the bottom, you break that out into a new function. In both of those cases, you can replace the original function call with a call to the new "top half" function followed by a call to the new "bottom half" function. But if the similar parts are in the middle of the function, you can break them out into a strategy. This lets you replace a call to each of the specialized methods with a call to the new combined function, passing the appropriate strategy to handle the bit in the middle.
Consider the following two functions: