A string of pearls is like a necklace made up of lots of little pearls that are all connected together. In the same way, China's string of pearls is a bunch of important ports and naval bases that China has been building in different countries all around the Indian Ocean region.
Just like you might wear a necklace to look pretty, China is building these ports and naval bases to help trade with other countries and also to have more control over the oceans in that region.
China is making these ports and bases in places like Pakistan, Sri Lanka, and Myanmar, and they have been doing this for many years. These ports and bases help China to be able to get resources more easily and also to have a backup plan in case anything goes wrong with their current ports.
Some people worry that China is using these ports and bases to control the countries they are building them in and the ocean region itself. This is why some people are keeping a close eye on what China is doing with its string of pearls.