A destination tag is an optional, unique number assigned to your wallet address for XRP, XLM and USDC (Stellar). This is necessary for a centralized exchange such as PDAX to be able to identify your wallet and credit it accordingly.
To prevent loss of funds, double-check that you are using the correct destination tag when sending to OR from your PDAX wallet.
To get your destination tag on PDAX, simply view your wallet address for any of the assets that support a destination tag. Please note that you may have a different destination tag for each asset.