Revelation 6:9 And when he had opened the fifth seal, I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held:
10 And they cried with a loud voice, saying, How long, O Lord, holy and true, dost thou not judge and avenge our blood on them that dwell on the earth?
11 And white robes were given unto every one of them; and it was said unto them, that they should rest yet for a little season, until their fellowservants also and their brethren, that should be killed as they were, should be fulfilled.
The fifth seal starts us seeing the saints that were slain for God in all that has happened. These are ones that are saved in the tribulation by the 144000 that preach and they cry out to God how long o Lord are You going to let this go on. But the Lord says to rest a little while till all that are saved have joined them. They are all given white robes to wear signifying they have been washed by the blood of Jesus.