Can refer to something that is straight or level.
El carpintero se aseguró de que la mesa estuviera perfectamente aplomado antes de terminar el trabajo.
The carpenter made sure that the table was perfectly plumb before finishing the job.
Para asegurarte de que la estantería esté aplomada, utiliza un nivel antes de fijarla a la pared.
To ensure that the shelf is plumb, use a level before securing it to the wall.