The tile selection window displays a grid of the active tile set. As it is a dockable window, you may move the window to any edge of the main window or have it floating anywhere on any screen.
Using the icons to the left, you can change the zoom level to suit your preference.
Click on a tile to make it the currently selected one (indicated by a dashed square surrounding the tile)
Double click a tile to edit it in the Tile Editor.
You can move tiles around in the grid by clicking and dragging to a new position. This will also update any maps and animations with the new indexes.
Holding shift while dragging a tile will replace the tile it is dropped on with a copy of the first.