From Yu-Gi-Oh!

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.

[edit] Level 4 or lower monsters

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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.

[edit] Level 5 or 6 monsters

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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.

[edit] Level 7 or higher monsters

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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.