A Transfer of Residence number is a way to help clear your baggage through UK customs without being charged duties, provided you meet certain criteria.
If you are a UK resident who has been outside of the UK for more than 12 months, or if you are a non-UK resident who is relocating to the UK you are eligible to apply for a TOR and you can find the link to the application here - https://www.gov.uk/guidance/application-for-transfer-of-residence-relief-tor1
If you are a UK resident who has been outside of the UK for less than 12 months, HMRC may be able to clear your baggage as a returning goods shipment if you have brought all of the items from the UK initially, and are simply shipping them back to the UK (if you have bought anything from outside the UK and include this in your baggage, your shipment will not qualify for this and you may be charged duties).
It can take some time for you to receive your TOR number back from HMRC once you apply for this online, so we would always recommend trying to apply for it prior to booking with My Baggage so that you can include this on your customs forms online.
If you don't receive your TOR number back in time before your shipment clears UK customs, you may have to pay the duties initially and then you can apply for a refund by completing a C285 form linked here - https://www.gov.uk/guidance/how-to-apply-for-a-repayment-of-import-duty-and-vat-if-youve-overpaid-c285