Jokers work by dragging them onto the play area and stacking cards on top of them according to the rules that each Joker has. There is a section in the game page that describes what each Joker can do. For example, The Black Palette Joker allows you to stack any black suite card on top of it in regular numerical order.
You can also drag the jokers around on the gameboard if you need to access the stack of cards underneath them.
In this screenshot, you can see some of the jokers (blueish cards with a J visible) are on the board with cards stacked on top of them.