A co-signer is someone who will also sign the contract you have with Housing Hand. They may be someone who does not wish to accept the full financial responsibility of being a guarantor, or they may not meet the requirements to be your UK rent guarantor directly.
Your co-signer is a second point of contact and is also jointly liable to repay Housing Hand for any monies paid out on your behalf if you default during your tenancy. They must have an income, however their credit history is not relevant. If you are from overseas, your co-signer does not need to be based in the UK so can be a parent or friend in your home country.