The overall meaning of the word overall is that it covers all the parts of something. For example, the individual parts don't matter as much as the overall value.
The word overall is the sum of its parts. Take the word over, meaning "above," and add it to the word all, meaning "everything" and you get overall, meaning "above everything". But remember, it's an adjective as in the overall impression of the music, the overall effect of the new tax code, the overall direction (as opposed to individual little changes.) Remember that overalls, the clothes you wore as a kid, went "over all" your other clothes.