Clicking a button will activate that tab. Pressing Shift-Left and Shift-Right will switch to the tab left or right of the current one, while Shift-Down creates a new tab. Pressing Ctrl-Left and Ctrl-Right will renumber the current tab by moving it to the left or to the right.
The tabbar itself can be configured similarly to a normal terminal, but with a resource class of "URxvt.tabbed". In addition, it supports the following four resources (shown with defaults):
URxvt.tabbed.tabbar-fg: <colour-index, default 3> URxvt.tabbed.tabbar-bg: <colour-index, default 0> URxvt.tabbed.tab-fg: <colour-index, default 0> URxvt.tabbed.tab-bg: <colour-index, default 1>
See COLOR AND GRAPHICS in the @@RXVT_NAME@@(1) manpage for valid indices.