According to Mark, this verse means that after the soldiers made fun of Jesus and dressed him in a fancy purple robe to laugh at him, they took the robe off and put his regular clothes back on, then took him away to nail him to the cross. Christians believe this part shows how much Jesus was hurt and teased before he was put on the cross for everyone.
Mark 15:20: And when they had mocked him, they took off the purple from him, and put his own clothes on him, and led him out to crucify him.
Mark 15:20 describes the soldiers mocking Jesus with a purple robe, which aligns with John 19:2, but here it specifies the crown of thorns was given alongside the robe, which Mark 15:20 does not mention while leading Jesus away.
John 19:2: And the soldiers platted a crown of thorns, and put [it] on his head, and they put on him a purple robe,