Sea ice is the frozen surface of the ocean. The amount of the ocean's surface covered by sea ice varies throughout the year - it forms in the winter and partly melts away during the summer - but it also varies from year to year.
Since 1979, scientists have been monitoring the area covered by sea ice using satellites. They have seen that this area - called sea ice extent - has reduced year on year, and that this reduction is accelerating.
You can see how much sea ice is in the Arctic Ocean RIGHT NOW by looking at the maps published daily on the website of the National Snow and Ice Data Centre in the US: