Apply pricebook items to line items
Your pricebook holds the services and items you sell. Applying one means adding it to a line item on an estimate, an invoice or a Work Order, instead of typing the description and price by hand. The line is linked to the pricebook item you chose, so the same item can be used on any of the three.
When you are building an estimate, you can browse the pricebook to pick items.
Applying a pricebook item is separate from changing the price on a line. Overriding a price is its own action, and so is maintaining the pricebook itself.