The critical path is the set of functionalities that a typical user would execute on the system in their daily use.
These are the features that the largest number of users will use the most, so if they do not work, the impact will be very high.
What are those functionalities? They will depend on each system.
Frequently asked questions
What is the critical path in testing?
The critical path is the set of features that a typical user would run on the system in everyday use.
Why is it important to test it?
Because they are the features most users will use the most: if they don't work, the impact is very high. Which ones they are depends on each system.
Keep reading: I don't have time to test everything and How to choose which platforms to test on?. If you want to go deeper, I recommend the course Risk management.