Level

A Monster Card's Level is a rough indication of how powerful that card is, represented by a number of stars in the top-right. The maximum Level a monster can have is 12, and the Minimum is one. A Monster Card cannot be treated as Level 0, even with an effect, the minimum will always be Level 1.

Level 4 or lower monsters
Can be Normal Summoned to the field, unless a card condition specifically prohibits it, and/or implies a special condition in which it must be Summoned another way, for example "Rare Metal Dragon".

Level 5 or 6 monsters
Can be Tribute Summoned by Tributing 1 monster on your side of the field, unless a card condition specifically prohibits it, and/or implies a special condition in which it must be summoned another way.

Level 7 or higher monsters
Can be Tribute Summoned by Tributing 2 monsters on your side of the field, unless a card condition specifically prohibits it, and/or implies a special condition in which it must be summoned another way.

You must only Tribute 3 monsters if the card allows it (such as to unlock the effect of "Moisture Creature"), or the card's Summoning Conditions require it (such as with "Victory Dragon").

As of the release of the s in the  anime, there are now Negative Levels.