Duration wears two hats. As a time measure, macaulay-duration is the weighted average number of years until you get your money. As a risk measure, modified-duration is roughly the percentage price change for a one percentage point change in yield.
Traders use the second meaning almost exclusively. Duration is the single number that tells you how much rate risk a portfolio carries, and it is additive: the duration of a book is the value-weighted average of its parts.
Example: a portfolio has a duration of 6.2. Yields rise 40 basis points. The portfolio loses roughly 6.2 x 0.40 = 2.5%. On $5,000,000 that is $125,000, before the small offsetting help from convexity.
Related: macaulay-duration, modified-duration, effective-duration, dv01, convexity