You can add a discount by either selecting a "discount" type of product from your Products dropdown on the quote or order, or by manually adding a discount (the option next to the products dropdown), and enter the amount in the negative form (-50) and then you'll see the option to make it a $ or % discount.
Discounts vs. Rebates
When entering a discount on a quote or order, you have the option to set it as Pre-Tax. If you select Pre-Tax, it will make it a standard "pre-tax discount", taken off the order's total before any sales tax is applied. If you leave Pre-Tax unchecked, it will be issued as a "rebate". Rebates are discounts taken after tax instead of pre-tax, so sales tax is calculated based on the original order's total before taking off the discounted value. Here's a quick visual of a rebate vs. a pre-tax discount: