It must be Effective (Doing the right thing)
Can users complete their goals when using your product?
Any task should always be easy to understand and discover within a reasonable amount of time.
It’s really important that the experience of using your product matches with previous experiences users had, avoiding frustration or the need to learn a new system.
Main Metrics:

• Completion Rate
• Number of Errors

Must be Efficient (Doing the thing right)
How much effort users need to complete their goals?
Important things are the number of steps to complete a task; how long each task takes; how familiar users are with a system and how much users have to learn to be able to complete their goals. .
Main Metrics:

• Task Time
• Number of Steps to complete

Must be delightful (Satisfaction)
No matter how effective and efficient your product is, if it does not spark joy people won’t use it. And this is how you increase engagement: A great product that looks nice, that feels right and its pleasant to use. .
Main Metrics:

• Standardized satisfaction questionnaire
• Retention Rates