Monster Zone

Monster Card Zones are the 5 Boxes in the Playing Field (the below picture) with "Monster Card Zone" on them.

You can only have in play at any time 5 face-up and/or face-down cards for each of those Zone. Normally, Monster Cards (including Token Monsters) are placed there, but sometime Spell and Trap Cards are placed there if they treated as Monster Cards by some effect.

If all 5 Monster Card Zones are occupied, then you cannot Summon or Set another monster, unless you use the monsters you have as a Tribute or a Material Monster needed for the upcoming monster. Some cards, such as "Ojama King", "Ojama Knight" and "Ground Collapse" affect the number of Monster Card Zones that are usable.

Normally, you cannot place more than 1 Card in 1 of those Zone, but with Xyz Summoning you can do so, by placing the Xyz Material you are using underneath the Xyz Monster you are Summoning.