Brisk is an adjective that refers to something marked by speed or energy. A brisk tea, for instance, is an invigorating tea. If you walk briskly, you are walking fast or vigorously.
The sound of her iron shoes upon the hard road was quite musical, as she came along at a much brisker trot than usual. (Charles Dickers – Great Expectations)
in my opinion brisk walk mean don’t loose your shoulders, have a active walk that adds plus pints in your personality.