Generator expressions in Python are a concise and efficient way to create iterators. They allow for the generation of values on-the-fly, which means that values are produced only when needed, rather than being stored in memory all at once. This lazy evaluation makes generator expressions particularly useful for handling large datasets or streams of data. The syntax is similar to list comprehensions, but instead of using square brackets, generator expressions are enclosed in parentheses. This feature not only enhances memory efficiency but also improves performance in scenarios where only a portion of the data is required at any given time.