When clicked you'll notice two things happen.  The first will be the display of a bar showing all installed widgets installed on your Mac will appear at the bottom of the screen.  Just scroll through the widgets by clicking on the arrows to the left and right of the bar to see what's available.  The second will be the addition of a small x encapsulated in a circle on the upper left corner of all active Widgets.

Add a Widget
It's easy to add a widget to Dashboard.  Just drag it from the widget bar and place it anywhere you would like it to appear when Dashboard is launched.  You'll soon see a cool ripple animation as it is added.  You can also add a widget by clicking on it's icon in the widget bar.  Once clicked, it will be placed in the middle of Dashboard.  You can then move it where you like.

 

 

Manage Widgets - The Widgets Widget
Managing your widgets is easy.  When the Widget bar is active you will see an icon that says "Manage Widgets...".  When you click on it Dashboard will display a widget manager known as the Widgets Widget.  To disable a Widget, just uncheck the box to the left.  You can use the white scroll bar on the right to view all installed Widgets.  You can sort the Widget by Name or Date by pulling on the drop down menu on the top.  If you click on "More Widgets" you will be taken to Apple's Widget download page.

 

 

Remove or Hide a Widget
As show above, a small x is activated when the "Manage Widgets" button is clicked on.  To remove a Widget from Dashboard just click on the x and observe how the Widget disappears.  There is another way to remove and hide a Widget without usng the Widget bar.  Press the Option key and place your mouse over a Widget.  You will see the encapsulated small x appear only on that Widget.  Look at the calculator below: