Task-based learning takes a different approach to a traditional PPP (present, practise, produce) lesson. Students are given a task to do, for example order food in a restaurant, book a taxi etc, they could also be given a probem to solve. The teacher might decide to have the students listen to a recording of people doing the same task before students tackle it themselves.
Students do the task in groups or pairs while the teacher monitors. At the end of the activity, the teacher reviews what went well (or not so well). If a language area was a consistent problem or an error was made by various members of the class, the teacher can choose to focus on this immediately or in a subsequent lesson.
Language input takes place after completion of the task. Students can then be given a similar activity to do in order to practise and consolidate the language studied. The advantage of this approach is that you don't spend time working on language points the students have already mastered.
This is a very brief look at the approach. For more details and ideas, you can look at books by Jane Willis. The text book series "Cutting Edge" follows a task-based syllabus.