Fusion Tables was a Google Drive service used for visualizing large amounts of data, especially on a map. It started as a research project in Google Labs in 2009 and later grew into one of the few free tools of its kind. It became wildly popular with data journalists.
It worked by compiling data from different tables and presenting it in a joined visualization. The new version would update itself whenever the users entered new data in the original tables.
Fusion Tables was an easy solution for people who needed to express and track lots of location-related data but weren't skilled with complicated mapping software. It was widely used by private individuals, government departments, news agencies, etc. The shut-down of the service forced them to seek new ways of storing and sharing their sizable archives.