A bubble level, also known as a spirit level, is a tool used to determine whether a surface is horizontal (level) or vertical (plumb). It contains a sealed glass tube filled with liquid and an air bubble. When the bubble is centered between two marked lines, the surface is level. Bubble levels come in various sizes and types, including torpedo levels and laser levels. They are commonly used in construction, carpentry, and home improvement projects to ensure accuracy in aligning structures, such as shelves, frames, and walls.