Original: While a piece of Western music generally has a home key, a passage within it may modulate to another key, which is usually closely related to the home key, that is, close to the original around the circle of fifths.
Simplified: A piece of Western music usually has a home key, and a passage within it may modulate to another key, which is usually closely related to the home key. (Usually, close to the original around the circle of fifths.)