As a writer, I find it difficult to explain "tone" as far as my own work is concerned. I know beforehand whether the story will have a happy, bittersweet or sad ending, but all I can do is hope the reader picks up subtle clues along the way. I find that word choices have a lot to do with setting the tone. If the overall mood or tone of the story is somber or sad, I might use phrases like "She struggled to push aside the splintered old door...". The verbs and the adjectives all suggest a heaviness of tone.
If the overall mood of the story is positive, I might just use a different set of words: "She used all of her strength to break through that heavy wooden door.." It's the same situation as before, but the tone is more of a positive determination, not a struggle.