Depending on your setting, you either see a toolbar or a ribbon with many buttons. The buttons all show a tooltip explaining their function when you hover the mouse pointer over them.
On the left side of the window is a locator bar. This provides a quick visual reference as to where the changes lie within the file. The bar has three columns. The left column refers to the left pane, the right column to the right pane, and the centre column to the bottom pane (if present). In one-pane view only the left column is used. The locator bar can also be used as a scroll bar to scroll all the windows simultaneously.
If you double click on a word then every occurrence of that word will be highlighted throughout the document, both in the main panes and the locator bar. Double click on the word again to remove the highlighting.
If you click in the left margin, or if you triple click within a line, that whole line will be selected.
Below the bottom window is the status bar. This shows the number of lines added and deleted in
Mine, and the number of unresolved conflicts remaining. You can also see which line-ending style is in use, what encoding (ASCII, UTF-8, etc) is assumed by TortoiseMerge, and whether there is a byte-order-mark (BOM) present in UTF-8 files. You can change line-ending style and encoding by double-clicking the corresponding column of that view. A dialog box will be displayed. The left side of the status bar is used to display hints when you hover the mouse over a control.