When we have a desire to be good people 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 determine 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 (to use a very obvious example), you'd need to know whom to call, when they need help, where to go, what to bring. Armed with that, 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.
Based on this concept, it follows that 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.
The Writings also say that in a number of cases a "field" represents the doctrine of the church, which sounds markedly different. The desire for good is emotional, a drive, a wanting; doctrine is a set of ideas, dry and intellectual. But the Writings also say that for the church to be true, its doctrine must be centered on a desire for good and must lead people toward doing what is good. So despite how it sounds, the idea oif doctrine is actually closely bound up with the desire for good.