I've posted about editing before, and hinted it's a sometime series. This is the third in the series, on preserving the tone when you're editing your story.
The tone is the overall feel of a story. A story can be funny, gritty, light, dark, satiric or earnest, pessimistic, optimistic, fun, sad, or some shade in between. Many tones blend those elements.
The tone isn't necessarily in the individual words, though the words you choose to employ can affect the tone. Tone is an overall picture within the book, the lens you're showing your world through.
In your early drafts, you're probably not even aware of the tone you're writing in, especially if you're a pantster. If you don't know whether the ending is going to be uplifting, bittersweet, or sad, you don't know which details to highlight or how well the words need to trip across the page.
At some point in your macro edits, though, you do need to figure out what tone the story needs. Don't settle on a tone before you've figured out an ending. Everything about the story will have to lead to that ending, but the tone will make the ending inevitable, so that it's the only ending that works for the story.
Once you've figured out how light or dark, humorous or serious, hopeful or cranky your story is, then you need to tweak the story to reflect that tone. Pick details that support the tone. Pick language that implies the tone you're writing toward. Weather, the colors your perspective character notices, the events you highlight in depth, all lend themselves to the tone.
You'll probably find that a lot of scenes don't need much work, because you knew all along what tone you were writing. Those scenes are fine to leave as they are. It's the ones where you lost the thread, maybe thought too much about what you were writing, that you'll need to fix the tone.
Tone can tie together a whole story, connecting even seemingly disjointed vignettes. A reader can be carried through a rather confusing start if they feel it's all part of the same picture.
Similarly, though, if you switch the tone around, indicating carefree happiness in one scene, then following that up with a grisly murder, for instance, no matter how good your transition, you're likely to lose the reader.