When learners do an authentic task they are doing something that puts real communicative demands on them.
Example
The learners need to plan a trip around their country for a group of students on an exchange trip.
In the classroom
Authentic tasks are easy to identify as they are what we do with our own language all the time. Examples include phoning for information, preparing a party, planning a holiday, answering e-mail, preparing reports, holding a meeting and filling in application forms.
Further links:
https://www.teachingenglish.org.uk/article/six-types-task-tbl