Monster Card Zones (モンスターカードゾーン, Monsutākādozōn) are the 5 Boxes with "2" on them, in this picture. You can only have in play, at any one time, 5 Monster Cards and/or Token Monsters. If all 5 Monster Card Zones are Occupied, then you cannot summon another monster, unless you Tribute one or more of them first or use them for a Fusion Summon / Ritual / Synchro Summon/Xyz Summon.

Some cards, such as "Ojama King", "Ojama Knight", and "Ground Collapse" affect the number of Monster Card Zones that are usable.