How do you know what's there, if you are not local? Digitization in OpenStreetMap is based on satellite imagery. Depending on the project, different imagery is available. In some projects drone imagery is made available in order to be able to have a high resoluted and up-to-date base for digitization.
Is it necessary to have prior knowledge about mapping/digitization? You don't need to have prior knowledge in order to join the group. We are mappers of different skill levels and newcomers are always welcome!
What is OpenStreetMap? How to say it better than to refer you directly to the source? Visit https://www.openstreetmap.org/about to learn more.