Box number "1" in the above picture is your Field Card Zone (フィールドカードゾーン, Fīrudokādozōn). Only 1 Field Spell Card may be active at any time, and any new Field Spell Card that is activated destroys the other Field Spell Card which was active at that time, unless "Field Barrier" is on the field, in which case you can't activate a new "Field Spell Card". You may activate a Field Spell Card if there is one already present on your side of the field. By doing so, the first Field Spell Card is destroyed and sent to the Graveyard. A Field Spell Card may be also be Set here, instead of activated. If it is Set, and there is already a Field Spell Card on your opponent's side of the field, that Field Spell Card is not destroyed.
As of the new OCG rule changes, on March 21st, 2014, both players will be able to control a single Field Spell each.