Amazon Web Services (AWS) is a collection of cloud based web services you can use to built your applications. You'll be charged e.g. for the usage or on a monthly basis.
The Amazon team discovered the business potential of scalable cloud service solution when they faced scaling issues with their retail website in the early 2000s. The team developed different solutions to beat those scaling issues and opened them up to the public in 2006.
Today AWS is one of the largest cloud providers and powers a huge amount of well known internet companies (such as Netflix, Airbnb, Pinterest or Slack). Here are some case studies if you're interested how AWS helps those companies to scale their business.
The available services range from (virtual) servers (EC2), to storage (S3) up to devices you can use to test your apps (Device Farm).
If you want an overview of all the available services you should check out their product page which lists, structures and explains every service in detail.
Don't get too confused about the overwhelming number of services you can utilize. Understand that AWS is a service provider where you have access to different services you need while developing modern applications.
Sounds interesting but you don't want to spent a bunch of money monthly to play around and get used to their services? Amazon has you covered. There's a very attractive starter plan available which allows you to get familiar with their most prominent services for free (for a limited time).
We'd recommend that you'll sign up for the free account so you can use the full potential of the Serverless framework and explore the power of modern, serverless applications.