As well as wearing the backpack your worker can also carry one tool in each hand and drive one vehicle. Click on the Backpack tab to learn what tools the worker is carrying, move some stuff from the backpack or put something in it.
If you wish to use anything from the backpack, your hands should be empty. After that all you need to select a tool and click the button "Take".
To make the worker fulfill the task with a certain tool, move a tool into his hand. If the workers hands are full, first choose the unnecessary instrument, then press the "Put aside" button.