Lambda
Homepage
https://aws.amazon.com/lambda/
What is it?
Lambda is AWS' serverless computing offering. It doesn't actually mean 'serverless': it means you write some function, and then you can request Lambda to run it whenever you want, at which point it will provision a server to run the function and return the result.
It's a way to do various computing tasks on-demand, rather than provisioning your own dedicated server to sit around 24/7 waiting for you to use it, and allows for running the same function many times concurrently.