Under contract means the sellers have accepted an offer and the buyer is waiting on financing.
You could submit a backup offer in case buyer 1 does not complete the sale for any reason, you would be next in line. You will need to find a lender and get a pre approval letter before doing this. You can find a reputable lender and get a pre approval quickly,
I like The Lenders Network they're usually able to find you a lender that'll give you a approval letter quickly .