In Chinese philosophy there is the concept of Yin and Yang. Yin and Yang are drawn as the sunny and shady sides of a slope. They represent the polar opposites of what our world is composed of: hot and cold; fire and water; high and low; outer and inner; male and female.
Nothing is inherently Yin nor inherently Yang, for everything contains aspects of both.
Generally the exercises that we do in the west to 'stay fit' are what are considered to be Yang exercises. They are vigorous and energetic; work the muscles and make them bigger; cause you to get exhausted, sweat and hot.
There is nothing wrong with this type of exercise but we have to understand that they are not inherently nourishing. This is why, contrary to popular perception, athletes die younger than the general population; they have exhausted the body.
The Chinese were aware of this and so devised nourishing exercises which they called Qi Gong - the literal translation is Qi Work. Instead of jumping around and flailing arms the exercises are slow, meditative and internally powerful.
The result is that over time awareness develops of the internal organs of the body, and this awareness starts translating into better internal health. Hence these are Yin exercises as they work on the inner body (not the muscles and bones).
One of the beautiful aspects of these exercises is that because they are nourishing rather than exhausting, you can do them at any time and it will make you feel stronger.
This is a video of Dr Keown performing a series of exercises called the Twelve (or Eight) Brocades. His 'technique' isn't perfect but that is beside the point. The point is to practice them!