The storage monitor can be used to see how much of 1 item you have, you will need to click an item into the Monitor, this can then be locked by shift right clicking. When this is unlocked right clicking the monitor will remove the item.
The Conversion Monitor has the same function as the storage monitor but you can also take items out and put items back into the system (if it is locked) by either right clicking with an empty hand or with the item in hand. The conversion monitor will always try and give you a stack of 1 item.
There is also a gas and a fluid version of these monitors that work in the same way.