Plain, Level Flat means a level plain, or piece of land that is devoid of holes, valleys, or mountains or hills. Metaphorically, it means rectitude.
There are many instances of “plain” in the Bible, and what the majority refer to is a type of land. This type of land was desired by farmers because there is a great difference or blessing between farming a hillside and farming a level field.
When a farmer farms his field, he needs to plow the field. The good seed is set in rows usually, and before planting the seed, the field is “broken up” so that the soil is lose and the seeds can be inserted, and the seeds can easily take hold of root and sprout up through the soil. This is how plants, seeds, grow the best. Afterward the farmer may see weeds sprout up in the field, and he again plows, but not the rows, but rather the space between the rows. With his plow, he essentially kills or “unplants” these weeds that are competing with the good seed. Afterward, he may take a hoe and walk through the field and again use the hoe to kill and remove the weeds that are actually in the row of good plants so they do not complicate planting and harvesting. This needs to be done when the plants are large enough to distinguish good plants from unwanted plants. So this is done BEFORE the weeds overtake the good plants.
A Field on a Hill
But the problem comes in when the ground is either too rocky, or too sandy (sand does not have the nutriments of good soil), or the field is at an angle. Plowing with a horse and plow on an angle is a cursed thing, because gravity always pulls the plow to one side (the lower) and that causes the farmer to hurt instead of help his plants.
Rectitude
So the concept is a level place, where there are no hills or very low spots. The low spots cause their own problems because the horse and plow can easily get into them, but getting out of them is very difficult and also causes the straight row to go wrong.
But how do we get the concept of rectitude from plain? The idea is simply that in a person (God is a person also), that if a person dealing with situations in life has a “bent” that is a motivation that often is hidden, then it adversely affects the process of dealing with life, and it causes an undesired outcome.
For example, if a person has a “hidden” motive that is avarice (coveting riches for himself), or angry and envy against a certain person or group of people, or they want a particular concept to be displayed (for example the US government is endorsing homosexuality), then what is done by the person is twisted. It is not straight, or there is no rectitude in their dealings.
God has no hidden agenda.
What God is needs to be contemplated very carefully. God tells us openly what He is, what He wants, His character, and what He wants from us. He deals with us openly, directly, and without any hidden agenda.
On the other hand, Satan is the father of lies. Everything Satan says, promises, or offers is always with some kind of deception, or “bent” towards the things that Satan wants but is not open and forthright about.
Plain, Level Flat