The Deep Storage Unit or DSU is one of the most useful storage blocks in Tekkit Lite.
Each DSU can only hold one type of item at any given time, the upside of which is that it can hold up to two billion (2,000,000,000) of the said item and, when broken, it will still contain all the items held in it.
Shift clicking items in your inventory will move them into the DSU, as will clicking and placing them in the 'IN' slots.
You can change which sides are input and output, by clicking on the 'IN' 'OUT' buttons. It will always attempt to keep a full stack of its stored items in the output slot and, if the item is stackable, will leave one in each of the two input slots so that it's automatically filtered when connected to pipes or tubes.
The 'Stored' section shows how many items are contained in the DSU.
Great setup and use video can be found at this address. https://www.youtube.com/watch?v=mqXR4lDdWFc