To sacrifice a permanent is to take it and put it into its owner’s graveyard. The concept is pretty simple, but there are obviously interactions with other aspects of the game that can cause confusion.
Sacrificing a permanent is not the same as destroying that permanent, so a permanent that is sacrificed cannot be regenerated. You can activate a regeneration ability before you sacrifice it, but that won’t change anything when this permanent goes to the graveyard.
Many spells and activated abilities require sacrificing a permanent as a cost. Paying this cost, just like paying any other cost, does not use the stack and cannot be responded to. When playing Goblin Grenade, you sacrifice a Goblin as you cast the spell. By the time anyone receives priority after this spell is played, this Goblin is no longer on the battlefield. In other words, you cannot respond to a player casting a spell that has a sacrifice as an additional cost by removing that permanent somehow to prevent it from being sacrificed.
As mentioned above, sacrificing a permanent is not destroying it. However, when a permanent is sacrificed, it does “die” and it will trigger all relevant abilities that trigger when a permanent goes to the graveyard. For example, Doomed Traveler‘s ability will trigger when it is sacrificed.