The Ready-to-Start indicator in Project Task and Salesforce Task tells you when the task is ready to be started based on the status of its Predecessors. For more information on this feature, please see related article here: Ready to Start.
If you would like an email notification to be sent to the assigned Resource when their task is ready to be started, you will need to configure a workflow. This help article will walk you through how to configure such workflow.
The Ready-to-Start field in Salesforce Tasks is a formula field that is designed to automatically reflect the value from the Project Task. Since it is a formula field that simply reflects the value from the Project Task, Salesforce does not consider it a field update when the checkbox is set to "true". Therefore, it cannot be used to trigger an email notification.
You will then need to create your own custom ready-to-start checkbox in the Activity object and then configure an email workflow that will fire off when this checkbox is "true".
Now that you've created your own custom field in Activity object, you will need to populate the API name in Custom Settings.
The next step would be for you to create an email template.
Please see: Create an Email Template in Lightning Experience or Email Templates in Salesforce Classic
Once you've created an email template, you will then need to configure an email workflow.