The lists of battles and tanks can be filtered for various purposes. You might want to look at all tanks of a specific tier, all battles of one tank, all battles within a certain period of time, etc.
The filtering options appear as context menus for both tanks and battles:
The first option shown as "Default" in the image above is a preset for what colums to show, in other words what data to display on the grid.
The next option is the filter for what tanks to display, this can be a preset and/or custom filtering
More options are available as well, just try them out!
Use right mouse button click on a row to get more options, in the image below it shows how to filter battles showing only a spesific tank.