An RMA is a Returned Merchandise Authorization. It’s sometimes called an RGA (Returned Goods Authorization). It depends on the company that creates the transaction.
A company that allows returns of their product for repair, refund, or replacement likes to know what to expect the arrival of. To that end, they will often create and maintain an RMA process, which, if it’s operating properly, can allow the folks in the mailroom (or the loading dock) to appropriately handle the packages they receive.
Typically, an RMA number, when plugged into the database, will pull up information on when the product was sold, when the customer called in with a problem, what the agreed-upon resolution is to be, and where the replacement/repaired product or refund is to be sent.
You say you mailed the product, and that it has a tracking number. Who issued the tracking number? The USPS? If they’re the ones showing it “in transit”, then you get to hold them responsible for losing it. Did you have the product insured? If so, the insured value is what you’re entitled to receive (generally speaking). If all you paid for was a signed receipt showing proof of delivery, you’re probably only going to be able to get your postage and extra services charges refunded.
No matter what you’re entitled to, the only way to go about getting it is to file a claim.