Summoning Rules are basic rules that dictate how the vast majority of monsters can be Summoned. They are primarily dictated by Level, but may also be influenced by inherent limitations in their Summoning Conditions.
Level 4 or lower monsters can be Normal Summoned to the field or Set unless a card condition specifically prohibits it and/or implies a special condition in which it must be Summoned another way, such as for "Rare Metal Dragon".
Level 5 or 6 monsters can be Tribute Summoned by Tributing 1 monster 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 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 activate the effect of "Moisture Creature") or the card's Summoning Conditions require it (such as with "Victory Dragon" or "Obelisk the Tormentor"). In the original manga, "ten-star/Level 10" monsters must be Summoned with three Tributes.