View EC2 Fleet on the plugin site for more information.
Amazon EC2 Spot Fleet
This plugin uses Amazon EC2 Spot Fleet to launch instances instead of directly launching them by itself. A Spot Fleet is a collection, or fleet, of Spot Instances, and optionally On-Demand Instances. The Spot Fleet also attempts to maintain its target capacity fleet if your Spot Instances are interrupted due to a change in Spot prices or available capacity. For more information, see How Spot Fleet Works.
You'll need an AWS account to use this plugin, you can get one at AWS.
Once you have an account, create an IAM user with sufficient permissions to launch Spot Fleets (Spot Fleet Prerequisites) and get its AWS credentials.
Then you need to set up a fleet that will serve as the build fleet for Jenkins. You can use the AWS console to launch it or the AWS CLI tools. Make sure that you specify an SSH key that will be used later by Jenkins.
Once the fleet is launched, you can set it up by adding a new EC2 Fleet cloud in the "Manage Jenkins" > "Configure System" menu of Jenkins.
You can specify the scaling limits in your cloud settings. By default, Jenkins will try to scale fleet up if there are enough tasks waiting in the build queue and scale down idle nodes after a specified idleness period.
You can use the History tab in the AWS console to view the scaling history.