An "Item" in ProWorkflow is a single component of a larger project. Items could be to-do tasks that require a staff member to work a set number of hours, which may involve Labour In/Out rates. Items could be materials used over the course of a project, which are more concerned with quantity and cost per unit. Items could also be Expenses, more based around a single fixed cost.
Whichever item type you pick, you'll be able to affiliate costs and returns with it, helping you calculate your profit for the wider phases and larger Project.
You can determine which Item Types you have access to, as well as how each item will be fiscally interpreted from the Item Types menu in Settings. To the right hand side of the page, you can set whether they should be measured in Time or in Quantity. This will impact the options available in the Qty/Hours field.
Any time you add an Item, you'll be prompted to choose an Item Type: