This may seem as a naive question, but how do advances work? Is the figure something the publisher predicts the author to sell?
Advances are usually based on the number of copies the publisher thinks the book will sell. If it's a potentially big book they'll increase their marketing and promotion budget to help ensure that the sales are high.
What if the author sells below that figure? Do you get your usual royalties?
An advance is an advance against royalties. So if the publisher thinks your book will sell 20,000 copies they'll work out how much that means in royalties, and that amount will be your advance (that's over-simplified but it gives you an idea). Then you'll not receive any further payments until your book has sold those 20,000 copies, because you've already been paid for them.
If your book then sells more copies you'll start receiving royalty cheques: they're usually paid twice a year by the bigger publishers who focus on print sales, but publishers which specialise in e-books usually pay them more frequently.
If your book sells fewer than those 20,000 copies you'll not receive any royalty payments--but you won't have to pay back any of your advance unless you signed with a shonky publisher.
Ive never had advances but a friend of mine was able to put money on an apartment with hers. I have subbed to some pubs who offer them and just curious.
Some publishers pay no advances. Some pay token advances. Bigger publishers tend to pay bigger advances, especially to authors of books they think are going to be big.
If you have an agent you're likely to be paid bigger advances.