Items
Inventory → Manage items. Each item has a SKU, a name, a cost and a reorder point. The reorder point is what makes Needs reorder on the main page mean something.
Warehouses
Manage warehouses if stock lives in more than one place — including a van, which for a trades business is a warehouse that moves.
Movements
Record movement for anything that changes the count: receiving, using on a job, transferring between locations, a write-off after a count.
View movements is the history — what moved, when, and who recorded it.
What the front page tells you
SKUs, Value of what you hold, and Needs reorder. Below that, on-hand by item and by warehouse.