Field
The field is the background of a flag, the color or colors behind any symbols. A plain red flag with a white cross has a red field. The term comes from heraldry, where it means the background of a shield.
Everything else on a flag is placed on the field, so its color sets the contrast for the rest. A dark field with a light charge or a light field with a dark one reads well from a distance. NAVA's advice to use two or three basic colors starts here.
In the flag maker, the field is the background layout. Pick a solid layout for a single-color field, or a striped one, then click any band to change its color. The flag colors meaning guide has sourced examples of what countries say their colors stand for.