It's milk tea with tapioca.
the "Milktea" here is thin tea with milk, which does not taste like British milk tea, which is stronger, but almost tasteless.
Then they have tapioca/jellied pearls in the bottom. They don't give you a spoon but a large straw, so you can aspirate the tapioca into your trachea more easily.
Wikipedia: Bubble Tea.
Most bubble tea recipes contain a tea base mixed with fruit or milk, to which chewy tapioca balls (known as bubbles, pearls, or boba) and fruit jelly (especially tropical fruits like lychee) are often added. Ice-blended versions are usually mixed with fruit or syrup, resulting in a slushy consistency. There are many varieties of the drink with a wide range of ingredients with some ingredients being mango, taro, coconut, and peach. The two most popular varieties are bubble milk tea with tapioca and bubble milk green tea with tapioca.
more here at Weibo, a Chinese site. which lists the ten best places to buy it in China.
apparently it is also a big hit there too.
however, looking at the discarded small straws littering the street, I suspect except for the trendy students, the locals still prefer the much cheaper fruit juice or buko juice, served in a plastic cup or in a bag with a straw from a local street vendor to drink.