This is the 17752nd most frequent Russian word.
украсят
"Украсят" means "will decorate."
Here, "украсят" is used to indicate that flowers will adorn the room, adding decorative enhancement.
Цветы украсят комнату, сделав её уютнее.
Flowers will decorate the room, making it cozier.
In this sentence, "украсят" describes the act of ornaments adding aesthetic value to the Christmas tree.
Эти украшения украсят вашу рождественскую елку.
These ornaments will decorate your Christmas tree.
"Украсят" in this context is used to express that paintings will embellish and enliven the empty walls.