Dorsiflexion is the movement between your shin bone (tibia) and the top of your foot (talus), where the shin hinges forwards on the foot with the foot fixed on the ground, or the foot hinges on the shin with the foot off the ground.
Adequate dorsiflexion is essential for walking, running, and any activity that involves a combination of the two. Adequate dorsiflexion with the foot fixed on the ground is between 20-30. A way to assess this without needing to measure the angle is to get into a half kneeling position and drive the front knee forward as much as possible. Measure the distance that the knee is in front of the toes. If this is less than 5 inches, your dorsiflexion is lacking.
To improve your dorsiflexion, you can use the same position that you used to test it. Assume a half kneeling position, and with a pole or wall for support, drive your front knee forward so you feel a stretch behind the ankle/shin. Hold this for 30-60 seconds.