Third party consent to a search occurs when someone who is not the accused gives their consent for a search to be done. Third parties may give their consent as long as they have sufficient legal authority over the place that is to be searched. It is also legal if the person does not have such authority but the officer who is given consent has good reason to believe that they did.
An example of a third party consent search would be if a man is suspected of a crime. Police come to his house and ask his wife if they can search. She has sufficient legal authority to allow them to search, so if she consents, they may search and they may use anything found against the man.