Destination tag is a 9-digit figure, unique to each hosted wallet. It works as a wallet address.
Note that Ripple wallets at GateHub do not have a destination tag.
A destination tag is extra information with a Ripple transaction, just like a reference with bank transactions.
It is usually requested from the sender by the funds receiver. It identifies the transaction, in case several transactions from several senders may be sent to the same receiving address.
Some exchanges like Bitstamp require the users to include a destination tag, in addition to the receiving ripple address, when the user makes a deposit to the exchange. In this case the destination tag identifies the user within the receiving exchange. Other services don't require the destination tag, but instead provide unique Ripple addresses for each user.
When users wish to withdraw funds such as XRP via the Ripple network, some services like GateHub and Poloniex provide the ability to enter the destination tag. This facilitates deposits to services where destination tag is required (like Bitstamp).
Transferring XRP from another service (Poloniex, Bitstamp, ShapeShift ...) to GateHub
When transferring XRP, just enter your Ripple address. Destination tag is not required; leave this field empty.
Transferring XRP from GateHub to Poloniex
Just enter your Poloniex ripple deposit address. No destination tag required.
Transferring XRP from GateHub to Bitstamp
Enter the Ripple address and destination tag provided by Bitstamp.