In the Bible, eating represents taking in knowledge and the desire for good. Trees represent knowledge and understanding that come from the Lord. So this verse says the people of the Most Ancient Church – "man" or "Adam" – were free to gather the knowledge that flowed to them from the Lord.

This happened in a way that we cannot truly understand now. Because the people of the Most Ancient Church were in a state of love to the Lord, they know instantly, from their affections, what was true. They didn't have to ponder logic and ask questions; they simply knew from their emotional response whether an idea was in accord with love to the Lord.

(Reference: Arcana Coelestia 0125)

A "field" in the Bible usually represents the Lord's church, and more specifically the desire for good within the church. It's where good things start, take root, and grow. When you have a desire to be a good person and to do good things, the natural first questions are "What does that mean?", "What should I do?", "What can I do?". You look for ideas, concepts, direction. Once you figure out something you want to do or a change you want to make in yourself, you seek specific knowledge. If you want to volunteer at a food pantry, say, you'd need to know whom to call, when they need help, where to go, what to bring. Armed with that knowhow, you're ready to get to work. That process could be compared to food production. You start with a field -- which is that desire to be good. Then you plant seeds -- those ideas and concepts. Those seeds sprout into plants -- the specific facts and knowledge needed for the task (easily seen in the food pantry example, but also true with deeper tasks like "being more tolerant of my co-workers" or "taking more time for prayer," or "consciously being a more loving spouse"). Finally, those plants produce food -- the actual good thing that you go and do. The Writings also say that in a number of cases a "field" represents the doctrine, or teachings, of the church. This sounds markedly different. The desire for good is emotional, a drive, a wanting; doctrine is a set of ideas. But for a church to be true, its doctrine must be centered on a desire for good, and must lead people toward doing what is good. So sound doctrine is actually closely bound up with the desire for good.