The word "plumb" has several meanings, depending on its context.
1) In a general sense, it means something is perfectly or exactly right, accurate or correct. For example, you might say that your measurements are plumb if they are precisely aligned with the vertical axis.
2) It can also refer to a type of woodwind instrument, specifically a flute or oboe, which has a conical bore and is commonly used in classical music ensembles.
3) In construction terms, "plumb" refers to being perfectly horizontal, level or upright. For example, if you were building a wall, you would use plumb lines to ensure that it was erected vertically and evenly.
4) Finally, in informal language, the word can be used as an adjective to describe something that is excellent or of high quality. You might say "That's plumb amazing!" if you were really impressed by something.