What Is a Level?

...
How Does a Level Work?
...

Levels are tools used to make sure surfaces are either level (parallel to the floor) or plumb (perpendicular to the floor). They are used in construction, masonry, and around the house for various projects.

Spirit Levels

...
When the bubble is centered, the object is level

Spirit levels are very commonly used in construction. The word spirit refers to the liquid that fills the vial used in this type of level. Bulls eye levels are the most common spirit levels, and consist of a large piece that looks like a ruler, with a small window in one or more parts of it that contains a small vial of liquid with a bubble in it. The bubble within the vial fits neatly between two lines drawn on the vial and when it is level with the floor the bubble is in the center. When things are not level, the bubble within the vial travels toward one end more than another, and is not center. To make something level, the level is placed up against it and it is shifted until the bubble lands in the center of the vial.

Laser Levels

...

Other types of levels are available that work on the same principle as the spirit level. However, recently laser levels have successfully come into greater use among those who wish to hang things in series. A laser level scatters a laser beam and projects it on a wall. These levels have a built in spirit level which electronically indicates when the line is parallel to the floor. The user can then hang a series of pictures or cabinets along the straight line and assure that they are level.