Depending on your point of view, mosses can be seen as very attractive and able to grow in conditions where grass cannot. It also has the added benefit of no added time and chemical inputs. There is information on moss here: http://extension.umd.edu/hgic/topics/moss-your-landscape
We do not recommend moss killer products. If you have moss but want grass, you need to correct the underlying reasons why the moss is there. Moss tolerates what grass cannot: dense shade, low pH (acid soil), compacted soil, wet soil, and low fertility. In order to grow grass, you'll need to change all of these conditions.
Start by doing a soil test. Then follow the recommendations for lime to correct the pH, and for fertilizer. In the future, your lawn should be fertilized every fall. Here is a comprehensive fact sheet that explains how to renovate your lawn and how to maintain it in the future: https://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/hgic/HGIC_Pubs/lawn_pubs/HG102-Rev-2018%20Lawn%20Establishment%20Renovation%20Overseeding.pdf
If you have a lot of shade, you will just be wasting your time and money trying to grow grass where it does not want to grow. Take a look at this: http://extension.umd.edu/hgic/shade-lawns
This is a good publication about moss and what you can do:http://pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/430/430-536/430-536_pdf.pdf