Choose the type under Discount value in each delivery discount.
Free
The rate becomes $0.
Percentage
Takes a percentage off the rate, for example 50% off makes $12.00 into $6.00. The maximum is 100%.
Fixed Amount Off
Takes a set amount off the rate, for example $5 off makes $12.00 into $7.00.
If the amount is more than the rate, the rate becomes free. It never goes below $0. This makes it ideal for a "shipping credit" such as "$25 off shipping".
Fixed Price
Sets the rate to a flat price, for example $5 makes $12.00 into $5.00.
Rates that already cost $5 or less are left alone. Fixed Price never increases a rate.
Other currencies
Fixed amounts and fixed prices are entered in your store's currency. For customers paying in another currency, they're converted automatically using Shopify's exchange rate. See Selling in multiple currencies.
What customers see
At checkout, the discounted rate shows the original price crossed out, with your discount's title (or code) underneath.