What is the meaning of tidy sum?

noun

• Often followed by `of'. ( a large number or amount or extent)
  • usage: "a batch of letters"; "a deal of trouble"; "a lot of money"; "he made a mint on the stock market"; "see the rest of the winners in our huge passel of photos"; "it must have cost plenty"; "a slew of journalists"; "a wad of money"
  • synonyms: batch, deal, flock, good deal, great deal, hatful, heap, lot, mass, mess, mickle, mint, mountain, muckle, passel, peck, pile, plenty, pot, quite a little, raft, sight, slew, spate, stack, wad