To lessen is to make smaller, to reduce, or to weaken. A high school football coach might try to lessen head injuries by improving the quality of the team's helmets.
If your cold symptoms lessen, it means you're sneezing and coughing much less and can probably go to school. If the value of your Pez collection lessens, it means those plastic candy dispensers can't be sold for quite as much money on eBay. And your friend might recommend that you take a yoga class or learn to meditate, especially if she thinks you should lessen your stress and anxiety.