Chinese coast guard authorities said it took "necessary control measures" and drove away a Japanese fishing vessel that had entered "territorial waters" around Senkaku Islands, state media reported.
Senkaku islands, there’s pretty regular posturing from both sides over the ownership of a couple barren rocks since they have implications for territorial waters.
I always find the most revealing map to give you an immediate view of the situation, to be that one the BBC made long ago, and keeps posting whenever this issue is brought back because of another incident. In blue, every country different claims. In red, China's claim.
Japan, just like China, has disputes over several groups of "islands" with several of their neighbors (at least Russia, South Korea, China and Taiwan). A very small number of these islands are inhabited. But many are more or less just rocks that formed from underwater volcanoes. (Japan grows by a few more islands regularly due to those.)
The islands this is about are claimed by Japan, China and Taiwan. They hold strategic significance, but it's mostly about suspected resources at the bottom of the sea.