A "string of pearls" is a term used to describe a series of military-related Chinese built installations in the Indian Ocean region. These installations are positioned along the Indian Ocean, and are believed to be strategically positioned to encircle India and its allies.
Think of it like a necklace made up of different beads - the beads being the military installations, and the necklace being the Indian Ocean.
These installations consist of ports, airfields, communication facilities, and radar stations. They are believed to be strategically placed in countries like Pakistan, Sri Lanka, Myanmar and Bangladesh, allowing China to maintain a foothold in the region.
The presence of these installations in the Indian Ocean has been a cause of concern for India, as it feels China's attempts to encircle the country would lead to an imbalance in power in the region.
Think of it like a game of capture the flag - if China is successful in capturing all the flags (or, in this case, setting up all these installations), India could potentially lose its influence in the region.