AWS Spot instances are unused instances that are available for a discounted price. They are a great way to save money on your instances. However, they are not guaranteed to be available and can be terminated at any time. Brev accounts for this by making sure your work is backed up and easy to start again.
Use a Spot instance
From the create instance page:
When you're picking the machine type, turn on the "Spot" toggle.
This will automatically default spot instances for any instance selected. When you pick a region, all the AWS regions that support spot instances will be available.