Expanding fields is a way to maximize the efficiency of your farm and decrease your work. You can either expand your field or join it with another.
To join two fields you need to own two fields close together. Simply connect one of your tractors to a plow and allow it to create fields by pressing the "O" key. Then plow the area you wish to turn to a field. Once you've done that, you're done!
You can "unplow" joined or expanded fields just by sowing grass (i.e., the road between 15 and 16). And if you want to remove the grass thereafter, the plow will do the work just fine.
Note: Hired workers will notice expansions/connections and will still work on them. You also cannot hire a worker to expand your fields. Any connections or extensions to a field will not appear on the map.