This Bible quote is from Revelation 7:14 and refers to those who have been saved by Jesus Christ through His sacrifice on the cross. The "blood of the Lamb" is a metaphor for Christ's sacrificial death, which cleanses believers of their sins and makes them white and pure in God's sight.
In a spiritual sense, believers "wash their robes" by repenting of their sins, seeking mercy and forgiveness, and placing their faith in Jesus Christ for their salvation. This act of repentance and faith is what allows the blood of Christ to cleanse and purify them, making them righteous in God's eyes.
