The field is a food source that can be worked by 5 people, with women having a higher gather rate. A field cost 100 wood and take 55 seconds to build. it provides infinite food if worked but only gives 50 if destroyed.
It's is worth checking the surrounding area before building a field, as some maps have them by default. The more workers you have on a field, the less efficient they get meaning if there is a lot of space available it might be applicable to build more fields with few workers instead of maxing out your fields with 5 workers.