When you zoom in on a block you can choose what you want the block to do by selecting one of the operations from the top most drop down list.
Each operation requires some inputs. These inputs will change what the block will output. For information on what each input does, hover over the text box.
Inputs can be mapped from the outputs of any blocks that are connected to the zoomed in block. You can do this by selecting the incoming block from the left hand side drop downs. As blocks often output lots of data you then need to select which one will be mapped to that input. Do this by selecting it from the 2nd drop down. If you want to enter a value yourself select "Custom" from the first drop down and then in the text box next to it, enter the value.
To zoom back out either press ok or click on the wrench icon in the upper left.