A secondary city is a place where people live and work that is not as big or famous as a big city but is still important.
Imagine you're playing a game of blocks. The big cities are like the giant towers that you build that everyone can see from far away. But the secondary cities are like smaller towers that are still really important even though they're not as big.
In these secondary cities, people still have jobs, they go to school, they shop and they have fun. They might not have all the fancy things that you can find in a big city, but they still have everything they need to live their daily lives.
Some examples of secondary cities are places like San Jose in California, Kyoto in Japan or Liverpool in England. They might not be as famous as cities like New York, Tokyo or London, but they are still really important for the people who live there. So, just like in your block game, having a lot of secondary cities is important to create a world with many places to live and work.