A property is a key-value pair associated with an entry. Properties can be set so they are associated with a single entry, with every entry in a tree, or with the whole buffer.
There are two main applications for properties in Org mode. First, properties are like tags, but with a value. Imagine maintaining a file where you document bugs and plan releases for a piece of software. Instead of using tags like ‘release_1’, ‘release_2’, you can use a property, say ‘Release’, that in different subtrees has different values, such as ‘1.0’ or ‘2.0’. Second, you can use properties to implement (very basic) database capabilities in an Org buffer. Imagine keeping track of your music CDs, where properties could be things such as the album, artist, date of release, number of tracks, and so on.
Properties can be conveniently edited and viewed in column view (see Column View).