'Robes' in the Bible

After these things I looked, and this is what I saw: a vast multitude which no one could count, [gathered] from every nation and from all the tribes and peoples and languages [of the earth], standing before the throne and before the Lamb (Christ), dressed in white robes, with palm branches in their hands;

Then one of the elders responded, saying to me, “These who are dressed in the long white robes—who are they, and from where did they come?”

I said to him, “My lord, you know [the answer].” And he said to me, “These are the people who come out of the great tribulation (persecution), and they have washed their robes and made them white in the blood of the Lamb [because of His atoning sacrifice].