SOの回答が非常にわかりやすいので、適宜翻訳して生きていこう。
タスク実行ロール
Referring to the documentation you can see that the execution role is the IAM role that executes ECS actions such as pulling the image and storing the application logs in cloudwatch.
タスクロール
The TaskRole then, is the IAM role used by the task itself. For example, if your container wants to call other AWS services like S3, SQS, etc then those permissions would need to be covered by the TaskRole.
