the.com/spirit level

means A tool with a liquid-filled vial containing an air bubble, used to check whether a surface is perfectly horizontal or vertical.

from The 'spirit' here isn't ghostly — it's the old word for a distilled alcohol-based liquid (as in 'spirits'), which is what early versions of the tool's vial were filled with instead of plain water, partly because alcohol resists freezing and has low viscosity, letting the bubble move freely and settle precisely. The device dates to the 17th century, and the name simply stuck even as manufacturers moved on to other fluids.