You define the components used in your assembly in the Product Edit for the assembly.
Code is the Product Code of the component.
The Description is displayed to ensure you have the right product.
The Price used by the component within this assembly is shown. If you over-ride the normal product price this will have a * next to it.
Qty indicates what quantity of the component is needed for each assembly.
If the Hidden box is checked the component is hidden from the customer.
The components are displayed in the order shown. You can change this order using the arrow buttons to move a row up or down.
The 
button adds a new component...
Enter the Product Code in the Code column. If the product exists the description and price will be updated.
To over-ride the Price check the box and enter a new price in the Price column.
The Quantity defaults to 1 but you can make it any appropriate value.
Check the Hidden box if you do not want the customer to see this component.
Click the 
if you have changed your mind and do not want this component.
Use the 
buttons to change or remove existing components.
These options control the way components are used by the system.
Price Rollup: If this is checked the total price charged to the customer is the sum of the assembly price plus all of its components. If it is unchecked the assembly price only is used.
Weight Rollup: If this is checked the total weight of the product is the sum of the assembly weight plus all of its components.
Component Only: Checking this box will prevent this product from being sold on its own. It will not appear on any product lists and cannot be added to the shopping basket except as part of an assembly.