A Tribute Summon (Japanese: アドバンス召喚Adobansu Shōkan "Advance Summon") is a form of Normal Summoning a Level 5 or higher monster by Tributing another monster(s) you control. Level 5 and 6 require one Tribute, while Level 7 and higher require two Tributes.
Tributing a monster for a Tribute Summon is a cost. Therefore, if the Tribute Summon is negated, the Tributed monster(s) won't return to the field.
Some cards can avoid the Tribute Summoning rules. For example, "Fog King" can be Tribute Summoned with one Tribute, or "Normal Summoned" without a Tribute. "Mausoleum of the Emperor" allows you to payLife Points instead of Tributing monsters, but the Normal Summon then cannot be considered a Tribute Summon.