The Runner Info window is only shown for UK and Irish Horse Racing markets. When the window is rolled up the header displays the number of different bits of information the window contains.
Runner Info window dockedWhen expanded the window displays various bits of information about the selection that your mouse is currently over such as the Jockey Name and Trainer Name. By right clicking on the header it brings up the various options for this window.
Runner Info window floating with Right click options
Runner Info window undocked with horizontal and vertical grid lines displayed