Secondary data refers to information that was collected by someone else (an individual, an organization, a government agency, or a research institution) for a primary purpose different from your current research question, but which can be re-analyzed and re-purposed to answer your unique research inquiries.
Think of it this way:
Primary Data: If you conduct a new survey specifically designed to understand current consumer preferences for sustainable packaging, that's primary data. You collected it for your immediate research need.
Secondary Data: If you instead obtain a data set from a national statistics agency that contains information on household consumption patterns over the last decade, and you then analyze that existing data to identify trends in sustainable purchasing, that's secondary data. The agency collected it for its own statistical reporting, but you're using it for your specific dissertation topic.