When you are using a database like MySQL, Postgres, or MongoDB, you provision a particular server to run your database. You need to specify your instance size, how many CPUs you need, RAM, storage, etc. before hand. This is not the case with DynamoDB.
In DynamoDB you just provision read and write capacity units. These units are used to allow a given number of operations/second. This way DynamoDB with a pricing model makes things simpler.