OpenStreetMap uses tags to add meaning to geographic objects. There is no fixed list of those tags. New tags can be invented and used as needed. Everybody can come up with a new tag and add it to new or existing objects. This makes OpenStreetMap enormously flexible, but sometimes also a bit hard to work with.
Whether you are contributing to OSM or using the OSM data, there are always questions like: What tags do people use for feature X? What tags can I use for feature Y so that it appears properly on the map? Is the tag Z described on the wiki actually in use and where?
Taginfo helps you by showing statistics about which tags are actually in the database, how many people use those tags, where they are used and so on. It also gets information about tags from the wiki and from other places. Taginfo tries to bring together all information about tags to help you understand how they are used and what they mean.