My experience is that in a refi any escrow refunds are simply extra dollar amounts that you borrowed, but probably shouldn't have. In an example, if an old loan had impounding with $4000 balance for taxes and insurance, your new loan will likely be increased by $4000 to start a new impound account with $4000 balance. Eventually you will get $4000 refunded from the former lender, which is money you borrowed in the new loan to fund the new impound account.
You have 2 good options. If you can get a better return on your investment elsewhere, put it there. If you can't get a better return, use it to pay down principal, decreasing your interest expense over the life of the loan. Less favorable options are to keep it at a lower interest rate (savings or checking account) or spend it, in which case you are paying interest on that over the life of your loan.