AWS Lightsail instances are t2 class EC2 instances. They are lightweight, simplified product offerings from AWS. They are monthly billed, thus, you incur cost even when you have the instance stopped. They have fixed size disk volumes. Though these VMs are derived from similar class as EC2 t2 class, the Lightsail has very limited subset of EC2 features and options.
The Lightsail has a simplified console with very limited functionality compare to EC2 dashboard. You can configure many of your network and routes, they are not as easy to configure as in EC2 dashboard.
Bandwidth is almost unlimited that is a significant monthly bandwidth allowance before you may incur charges.
The Lightsail console has a built-in SSH client. You can access your instances with a standard SSH client as well.
AWS does not actively advertise about Lightsail; however, this particular product offerings are competing with similar VPS offerings in the market. The offerings are similar in price and their features.