What is it?
A think aloud is a method while participant is using the product, you are asking questions. Participant think aloud simultaneous and share whatever they are looking at, thinking, doing, and feeling.
How can I use it?
You can prepare a script for the beginning part of the study. Ask as few questions as possible during the study and do not interrupt the participant. You may have to wait quietly, be patient.
When can I use it?
Especially when you you reach something testable, you can use it. Usability tests are the common usage area of this method.