In a nutshell the years begin at 1900 and go down toward the bottom. Frequency of play is from left (lower) to right (higher). As an example 1984 shows the Red Hot Chili Peppers as very frequently played by a lot of DJs.
Click on a map spot and it will bring up information about the artist, the number of plays and more.
A map of the favorite music of Last.fm listeners. Circles to the right of the map have more plays per listener. The Y axis shows chronology, with the oldest tracks at the top of the chart. The larger the circle, the more plays the artist has received.