Example - when renting a car, who is responsible for what? You are responsible for not damaging it, not speeding and paying tolls. The rental company is responsible for tire pressure, full tank of gas and mechanics. It is similar in the AWS cloud.
The shared responsibility model
You are responsible for the security in the cloud, while AWS is responsible for security of the cloud - eg. hardware and global infrastructure. Further AWS is responsible for providing reliable services for compute, storage, databases and networking. Customer data, identity and access management, OS, network and firewall, client- and serverside encryption and network traffic protection are all the responsibility of the customers of AWS - ie. you.
In general, can you do this yourself in the AWS Management Console? If yes, then you are likely responsible. If no, then it is typically the responsibility of AWS. Encryption is a shared responsibility.