What does indestructable mean in magic the gathering?

Answer 1

It means "destroy" effects don't work on it. Darksteel Colossus can't be shattered, disenchanted, wrath of God, Seal of doom, etc.

However since it can be sacrificed, cards like Chainer's Edict still work. Also reducing it's stats would work, so Mutilate can kill Indestructible creatures.

Damage or combat damage destroys creatures, so it wouldn't destroy an indestructible creature. However it does take the damage, but the damage doesn't kill it.

Answer 2

Indestructible creatures in Magic: the Gathering cannot be destroyed by lethal damage or destruction effects, but they are put into the graveyard if it has 0 or less toughness or is sacrificed. Exiling it, return it to hand, stealing control or put it into the library is also a way to get indestructible creatures out of the way. An indestructible permanant can also be sent to the graveyard by the Legend rule, or the Planeswalker uniqueness rule.