Tabs are a mechanism for allowing multiple panels of components to share the same space in a window. You control which of those panels is visible by selecting the tab for that panel. You can think of the panels as a stack of papers, one of which is on top of the stack. You can see the panel on top of the stack, but you can't see the panels which are underneath; to see one of the panels underneath, you need to raise it to the top of the stack, at which point the previous top panel will no longer be visible.
In JRaceman, the tabs are just below the menu bar in the main window. Select a tab by moving the pointer to that tab and clicking the left mouse button. The panel for that tab will appear.
JRaceman uses hierachical tabs; in other words, many of the panels themselves contain a set of panels controlled by tabs. When this kind of panel is made visible by clicking on its tab, another set of tabs appears just below the previously selected tab. You can then select one of those tabs to see that sub-panel. JRaceman uses three levels of tabs, so you may have to click three times (once at each level of tabs) to get to the desired panel.
To simplify finding the right panel, there is a Tabs menu which display the tabs in "upside-down" order from how they appear in the main window. In the main window, the third row of tabs ususally includes entries such as Edit, Query, Sheet, and Export. The Tabs menu includes these items as second-level menus within the Tabs menu, allowing you to navaigate directly to one of these panels without having to remember the first and second-level tabs required to see the desired tab for your panel.